I have so much to say about the halo lore but I'm on my phone so I can't say it now
Basically I can echo the sentiments that it's wonderful up until 4 came out and they basically had nowhere to go but into forerunner stuff.
Like, the books about the human/covenant war before the halo incident are great. I really enjoyed contact harvest.
My favorite halo book Is actually ghosts of onyx, because Kurt is a pretty cool character. I get the guys complaints about master chief because he's really not fleshed out in the games very well at all, and he's pretty boring in comparison to the rest of the Spartan-IIs that get covered in the books. It's always mentioned that John was never the best Spartan, just kind of lucky. I don't know if they've retconned that or not.
Also I think ghosts of onyx got retconned a little bit because it's basically entirely about the Spartan III program, which are what we saw in Halo Reach, but the IIIs in the book are nothing like the IIIs in the game.
Also I'm just now remembering that the Spartan program was initially conceived to fight human separatist terrorists. Man halo lore is neat.