Where can I find old versions of book.cls?

Download the file ftp://ftp.tug.org/historic/macros/latex2e/latex2e-19961201/base.tar.gz which contains the version of book.cls that you want. To actually generate book.cls you have to extract the archive, open a terminal into the directory containing all the files and run the command:

latex classes.ins

This command generates some files, among which the book.cls you want.

I've found version "1997/10/10 v1.3x" at ftp://tug.org/historic/systems/texlive/1998/TeXLive3/texmf/tex/latex/base/book.cls