This year's D paper had a lot of questions about Opposition and Appeal. The information in most legal books is difficult to read as there are not many dedicated legal provisions to group everything under. Also Appeals are not covered in the EPO Guidelines (except for Interlocutory Revision - Art. 109 EPC).
A good book for study and real-life is Proceedings Before the EPO: A Practical Guide to Success in Opposition and Appeal by Marcus Müller & Cees Mulder.
It not only explains the legal part, but it also explains how the procedures work and has a lot of practical tips. The version is from 2015, but most of it is still current.
For the Appeal part, also print out the Rules of Procedure Boards of Appeal (RPBA) as these are referenced quite frequently.
Note that the RPBA Rules are likely to be updated in the near future - see proposal here.