The Secret of the Five Words Classic (五字經訣) by Yáng​ Bān​hóu​ (楊班侯)

五字經訣

披從側方入，閃展全無空。
擔化對方力，搓磨試其功。
歉含力蓄使，粘沾不離宗。
隨進隨退走，拘意莫放鬆。
拿閉敵血脈，扳挽順勢封。
軟非用拙力，掤臂要圓撐。
摟進圓活力，摧堅戳敵峰。
掩護敵猛入，撮點致命攻。
墜走牽挽勢，繼續勿失空。
擠他虛實現，攤開即成功。

楊班侯

"When the opponent enters from sideways, I dodge and extend, nothing is completely empty.
Carry and neutralize the opponent’s force, file and grind to test the opponent’s skill.
Humbly preserve the force, use it conservatively. Attaching and adhering do not leave the center.
Follow to enter, follow to retreat and also to yield, keep your mind concentrated and do not take it easy.
Control and seal the opponent’s blood vessels, twist the arm to bend and seal following the situation.
Be soft and do not use the clumsy muscular power, in Peng the arms must be extended roundly.
When brush to enter, the power must be round and alive. Destroy the opponent’s strength and jab his sharpness.
Cover and protect where the opponent is entering and be fierce especially if he gathers force into a point and attacks a vital place.
Drop to yield and pull to save the situation and continue the attachment without losing it into emptiness.
Press him with insubstantial and substantial movements, once you are able to bounce him off balance, then you are successful."

Based on the translation by Yang Jwing Ming (楊俊敏) presented in the book Tai Chi Secrets of the Yang Style (太極拳楊氏先哲秘要)

Eight Secret Words of Technique (八字法訣) by Yáng​ Bān​hóu​ (楊班侯)

八字法訣

三換二捋一擠按，搭手遇掤莫讓先。
柔裡有剛攻不破，剛中無柔不為堅。
避人攻守要採列，力在驚彈走螺旋。
逞勢進攻貼身肘，肩胯膝打靠為先。

薑容樵

"Among three exchanges, there are two Rollbacks and one Push or Press. Once the opponent arms are attached and Ward-off is found, one should not allow the opponent to move first.
When there is hardness within softness, it is impossible to break through. If there is no softness within hardness, then it is not really strong.
To avoid the opponent’s offense and defense, one must use Pluck and Split. Power is manifested through a surprising spiral.
When in advantageous situation, advance closely to the body of the opponent and use Elbow Strike. When using Bump, shoulders, hips and knees are the first to be used for striking."

Based on the translation by Yang Jwing Ming (楊俊敏) presented in the book Tai Chi Secrets of the Yang Style (太極拳楊氏先哲秘要)

Green Way Research

Green Way research is the site I most hit at when looking for sources of texts or random topics. It usually comes out high on Google searches that are specific on topics of Tai Chi Chuan (太極拳), Qigong (氣功) or related internal martial arts.
In fact its scope is broader, and reduce it to a "martial arts ressource" would not to do justice to the research done by Mrs. and Mr. Garofalo on a range of topics that point to a way of life. For my purposes, which is looking for reliable references on Internal Martial Arts (內家武術), it is a very good companion. Probably the richest in English language. It works well as a starting point or as a second reference on many of the topics I work up for my personal study.
There is only one real downside: it does not make a selection or recommendation itself of the references it lists. The breadth of the references sometimes point to works of lesser interest or on occasions, even misleading. My understanding that this is by design: it is up to its readers and followers to make it own selection, determine their own study path.
Mr. Garofalo also publishes a blog called Cloud Hands.

Seeking calmness in the movements (動中求靜) from The Ten Important Points of Tai Chi Chuan (太極拳十要)

動中求靜

外家拳術，以跳躑為能，用盡氣力，故練習之后，無不喘氣者。太極拳以靜御動，雖動尤靜，故練架子愈慢愈好。慢則呼吸深長，氣沉丹田，自無血脈僨張之弊。學者細心休會，庶可得其意焉。

楊澄甫

"The fist techniques of the external styles grant expertise in jumping and walking, use the Qi and Li until it is exhausted. Therefore, right after practice, everybody will be panting. Taijiquan uses the calmness to control the movements, though moving remain calm. Therefore, when practicing postures, the slower the better. When slow, then the breathing is deep and long, the Qi is sunk to Dan Tian. Consequently, there is no harmful problem like the blood vessels' expansion. The practitioners should ponder and comprehend it carefully, then can (they) obtain its meaning."

By Yáng Chéngfǔ (楊澄甫), based on the translation by by Yang Jwing Ming (楊俊敏) presented in the book Tai Chi Secrets of the Yang Style (太極拳楊氏先哲秘要) and various translations on the internet

Mǎ Yuèliáng (馬岳樑) demonstrating Tuī Shǒu (推手)

The Xíngyì Song of Wújí (形意無極歌) by Jiāng Róngqiáo (薑容樵)

形意無極歌

混淪一氣內外修
涇渭不分至道由
空洞自然凝神靜
化虛還原此中求

薑容樵

"The unified chi of the original void is internally and externally cultivated.
The Jing and Wei rivers do not separate until they reach the source of the Dao.
The nature is empty, the spirit concentrate on quietness.
One seeks to become void and return to the origin."

From the book Xing Yi Mother Fists (形意母拳) and The Miscellaneous Form and Eight Postures Form of Xing Yi (形意杂式捶八式拳合刊) , by Jiāng Róngqiáo (薑容樵), based on the translation by Joseph Crandall

Thirteen Secret Words of Application (十三字用功訣) by Yáng​ Bān​hóu​ (楊班侯)

十三字用功訣

逢手遇掤莫如盤，粘沾不離得招難。
閉掤要上採列法，二把得實急無援。
按定四正隅方變，觸手即佔先上先。
捋擠二法趁機使，肘靠攻在腳跟前。
遇機得勢進退走，三前七星顧盼間。
周身實力意中定，聽探順化神氣關。
見實不上得攻手，何日功夫是體全。
操練不按體中用，修到終期藝難精。

薑容樵

"When encountering the opponent’s Peng (Ward-off) do not enter the territory. When this happens, to attach and adhere without separating is really difficult.
To shut off Peng, one must use Cai (Pluck) and Lie (Split). When these two techniques have become real, the opponent will be urgent to rescue.
An (Push) can be used to firm the four sides, consequently the corners have different variations. Once attaching with the opponent’s hands, immediately occupy the most advantageous position first.
Lu (rollback) and Ji (Press) should be used when the opportunities allow. When Zhou (Elbow) and Kao (Bump) are used to attack, the heels are ahead first.
When there is an opportunity and an advantageous position, advance forward and retreat backwards. Gu (Beware of the Left) and Pan (Look to the Right) are used within one third and two thirds rear of attention.
The solid power of the entire body depends on the Intention and Ding (Central Equilibrium). The skills of Ting (Listen) and probe (the opponent’s intention) follow and with neutralize are all related to the spirit and Qi.
When seeing the opponent’s firmness, I do not attack, but gain my offensive situation. The day that Gongfu can be accomplished is when the entire body acts as a complete unit.
If training without following the applications of the body, even if cultivating until the end, the art is still hard to refine."

Based on the translation by Yang Jwing Ming (楊俊敏) presented in the book Tai Chi Secrets of the Yang Style (太極拳楊氏先哲秘要)

Chinese Boxing: Masters and Methods by Robert W. Smith

Beyond being a pioneer book in introducing Chinese Martial Arts to western audiences, I have some other reasons to appreciate « Chinese Boxing : Masters and Methods » by Robert W. Smith.
First, it is clearly a personal account of Mr. Smith’s own experiences in Taiwan in the beginning of the sixties. It does not try to become a manual or guidebook: often western authors create handbooks from their personal experience; in most cases the results are sort of dogmatic books spelling intimate truths. Mr. Smith’s does not have that tone, and the accounts he brings are both interesting for the practice and inspiring.
Second, Mr. Smith does not slide into void Chinese explanations or new age mysticism (which is a criticism that even this blog could receive). He keeps his feet on solid ground and he would rather bring his subject to his own experiences, either relating them to his training in Judo, or to our own western cultural background. He looks for bridges so he can grasp the meaning, this can be seen on the many quotations he makes in the text (like on I particularly appreciate, G.K. Chesterton’s: “ I find that most round things are nice, particularly Eternity and a baby.“). In this book he is not a purist, he is looking to build his learning on his references.
Third, learning Chinese Martial Arts is a personal experience, and the accounts provided by Mr. Smith illustrate how the Arts. Some insight comes particularly from the chapter on Cheng Manching, the most extensive, where he reports some of the exchanges and methods of the master. For example, I recall reading that when students started to sweat, Cheng Manching (鄭曼青) would declare a pause on the training, which in itself is a lesson.
Mr. Smith would become a sort of role model for the “western that met a Chinese Master” that we can find from the end of the sixties on. Even though there are the good and the bad in this kind of figure, my best teachers were in the same position. This generation of western teachers is the bridge to the traditional knowledge and in my opinion the more they approach the style of Mr. Smith personal accounts in his book, the more valuable their transmission is.

Sūn Jiànyún (孫劍雲) performing Sun style Baguajian (孫式八卦劍)

The Song of Wújí (無極歌) by Jiāng Róngqiáo (薑容樵)

無極歌

人生太空，
無爭無竟，
意境渾然，
不著蹤影。

薑容樵

"People are created out of the great void,
Without striving, without contending,
The mind completely enclosed,
No trace is apparent."

From the book Xing Yi Mother Fists (形意母拳), by Jiāng Róngqiáo (薑容樵), based on the translation by Joseph Crandall