The extended tales
I recently bought and read the Alan Lee illustrated hardbacks of B&L, COH, FOG, FON, UT and TFTPR to accompany my TH, LOTR and TS.
They all look beautiful, but to be honest I regret them, especially for the price of around £25 per book. Each book contains the tale itself, then extracts of other versions and some notes of how it evolved.
Personally, I found that reading lots of slightly different versions of the tales took something away from them, turning them from feeling like real myth/lore into something that felt more cobbled together and less substantial.
Obviously, this is entirely my experience but I wondered if anyone else felt the same. I think it's better to stick with The Silmarillion and leave it there, so you have a set idea of the elder days.