Re: To buy, or not to buy...Budo Renshu
Ron is right, there are two different books, in fact, they are the only two books O'Sensei ever wrote. "Budo" is the book that most contemporary references are about. It has been translated into English and published in hardback in 1991, and then published in softcover by Kodansha in 1996. I do not know exactly how true-to-life the translation is, but the book does have an intro by Kisshomaru Doshu, so I would think it has his approval. There is also a book by Saito Sensei, "Commentary on 'Budo'", which was published in softcover in 1999. This volume attempts to explain or interpret Saito's take on the earlier work by O'Sensei.
The other book, "Budo Renshu" is more obscure. The 1978 translation by Larry Bieri is itself a rare collector's piece, which will set you back $400-500 if you can find one at all. It has accurate reproductions of the drawings which appear in O'Sensei's 1933 original, and text in both English and Japanese. It is put together in authentic Japanese fashion, with hand-bindery and back-to-front reading. This translation is the only one thus far of the original, although numerous references exist in works by other authors. I don't think any in-depth commentaries exist, like they do for "Budo".