So, I've got the 2nd Edition Core Book, read it, liked it.
Do I need to get *both* General's Handbooks, or can I get away with the newest one only? What is in each one?
GH 2018 should be all that you need, Wyddr. Each edition of the GH includes in-depth looks into Open, Narrative, and Matched Play respectively. The biggest change from book to book is the updated list of points cost for every unit in the game, at the back of the book. Allegiance abilities and battalions for factions that do not have a battletome or simply need more battalions are also included, but I think all of that stuff in the GH 2017 have been transferred into the newest 2018 Edition.
The only real change that I think doesn't translate between the GH 2017 and 2018 editions are some of the battle plans that games are based around. All of the battlepans with the new, refined mechanics are in the GH 2018. Such as heroes who are wizards or have artefacts can actually score on certain maps, etc.
Personally, I would just use the GH 2018, which is what I'm doing
.