Both books are very good. The Basic Taijiquan book teaches the fundamental 24-step set introduced to the masses in China back in the 70's, primarily to allow them to practice the art for health maintenance rather than as a fighting method.
Nonetheless, her instruction is very good as are the photos and explanations given. She also gives a fine overview of the art and discusses some important facets of the art and it's training.
The Advanced Taijiquan book is a presentation of the "Combined Form" which was introduced in the 70's as well, although it was intended for advanced practicioners and contains a full range of Taiji postures from various styles. Mrs. Mark discusses some advanced aspects of the art and includes details on various push-hands training methods.
I would recommend having these books, if for no other reason than to examine Mrs. Mark's flawless form and the fact that she is a legendary Taiji figure not only in the U.S., but in China as well.