The log books are books that were started when the rifles were either built or when they went through refurbishment. They contain all sorts of information showing things like when and to whom the rifle was issued, repairs made, bore wear checks, etc. The books are all written in Yugoslavian, however this link will show a translation to English.
www.asbrand.com/pics/guns/yugo_m59-66_logbook/ The Yugo M59, M59/66 were all produced at the Zastava plant. In the front of the book you will find a blue stamp. If the stamp has the word Zastava, then it is the original book issued to the rifle when the rifle was built, however most of the books have stamps that show the log books were created when the rifles were rebuilt. One common stamp is from TRZ Cacak. The TRZ Cacak is a repair and maintenance factory. If your book has any stamp on the first page other than the Zastava stamp, it would be a new book issued from when the rifle was rebuilt or when it was modified from a M59 to the M59/66.