marlon,
Yang Cheng-fu's book (The Essence and Applications of Taijiquan) was originally written by Dong Yingjie (Tung Ying-chieh) and called Taijiquan shiyongfa (Application Methods of Taijiquan). This was subsequently updated by Cheng Man Ching. This was confirmed by Yang Zhenji (YCF's 2nd son) when he wrote "Taijiquan tiyong quanshu was written by by fathers disciple, Zhen Manqing, according to my fathers performance narratives and requirements".
I agree with Randy Strausbaugh that Fu Zhong Wen's book is better. It has a more comprehensive coverage of each posture of the long form. Its not a sit down and read book, more a dip into reference book. However, both books are worth reading.
Very best wishes
.