The TEXbook is the definitive book documenting the TeX document typesetting system. It is written by Donald Knuth and published jointly by the American Mathematical Society and the Addison-Wesley Publishing Company. The latest edition of the book was published in 1996.
It is approximately 480 pages in length, in a spiral-bound format. The book has twenty-seven chapters covering basic topics, formatting mathematics, and some of the more esoteric features of the language. Each chapter includes loads of examples followed by some exercises designed to test what you've learned in that chapter. All the answers are in the back, unlike those stupid college books with just the odd answers.
There have been 26 printings of The TEXbook and the current printing is believed to be nearly error-free. Very few errors are known to exist but those that do are kept in a publically available list. Donald Knuth no longer works on the book though error reports are welcomed and will be met with a $2.56 reward, just like all his other books.