Most of the book covers the forms (Kibon, Taegeuk, Palgue, and the Black Belt ones). It has about 200 pages dedicated to forms (as a comparison, 13 pages to basic techniques - stances, blocks, strikes, and kicks). For more detailed descriptions of the "hows" and "whys" of each of the basic techniques, I'd recommend the "Taekwondo: The state of the art", ISBN 0-7679-0214-9.
Variations in the forms may appear in different associations (and Federations!), but since the "Official Taekwondo Training Manual" has the WTF endorsement, I'll consider it as WTF's official word for the forms.
Forms are shown not only with pictures, but also with feet transition diagrams in top view, drawn on the floor pattern you are supposed to cover during the whole form.
The most visible negative point I see in this book is that it doesn't include the Korean terminology for almost anything apart from the form names. All the basic techniques and form components are only named in English, and not always consistently (a fault, for an "official manual"). Example (in Taegeuk Yook-Jang): ... "execute a middle section round kick with the left foot". Then you go to the "Basic techniques" section, and there's no "middle section round kick", but a "roundhouse kick". On "Taekwondo: The state of the art" (which doesn't include the Korean terminology on the forms steps either, but does it on the basic techniques), for the same step: ..."left-leg turning kick". Then on its "Kicks" chapter you can find the "turning kick", and its Korean name, "Dollyo-Chagi".
My instructor (no book is replacement for one) uses the Korean terminology, and if we had that throughout the "Official Taekwondo Training Manual" in addition to the English names, that would be richer information.
Between this book and the "Taekwondo: The state of the art", I'd take both. The "Official Manual" as a more complete forms reference, and "The state of the art" for everything else.