A good way to find the differences is to look at the RFCs.
At the time a book published in 2000 was authored, the current version of BGP would have been according to the 1995 version of the standard: RFC1771. Today the current version of BGP is described by RFC4271 published in 2005.
RFC4271 has a handy list of the changes since the previous version in Appendix A:
(some of these changes were available as extensions before they were folded into the protocol update so the books may cover them or may not).
The top of the html version of the RFC also has a handy list of "updated by" references that you can use to find additional changes since 2005.
RFCs are written for implementors, so aren't necessarily the best way to learn how to use BGP; however they are the definitive reference for features so you can use them to identify what you want to learn about, and then use your google-fu to find more accessible documentation on that specific feature.