To answer my own post.
Found this handy guide on Goonhammer https://www.goonhammer.com/battletech-expanding-on-the-starter-boxes-books-and-inner-sphere/
And the Errata page on Catalyst. https://bg.battletech.com/errata/
Seems the split books are fourth printing of the original full volume for some confusing reason. They need to be used together and they are currently at 7th printing.
Judging by the erratas it's nothing major.