I just finished reading the first trilogy of the Horus Heresy series (my first dive into 40k) and enjoyed them quite a bit and feel like burning through some more over summer. Is the first trilogy the standard of quality for the entire Heresy series or does it go downhill from there?
Also are there any recommendations for fiction not involving Space Marines? I heard most of the Eldar stuff is pretty terrible.
Can't really help you with non Space Marine stuff, but for Horus Heresy, these are all worth reading:
Know No Fear (Dan Abnett unleashes his inner Michael Bay on the Battle of Calth and it's bitchin')
Scars and The Path of Heaven (White Scars get some characterization and are pretty cool)
Fear to Tread (Blood Angels vs Khornate daemons)
The Unremembered Empire (Guilliman builds a backup Imperium around Macragge while bickering with the Lion and fighting Konrad Curze)
Pharos (Loyalist Iron Warrior and an Imperial Fist being unlikely bros and Smurfs vs Night Lords)
The Buried Dagger (Death Guard falls to Nurgle)
Legion (Alpha Legion fucks shit up in Space Afghanistan)
Praetorian of Dorn (Imperial Fists vs Alpha Legion)
Vulkan Lives, Deathfire, and Old Earth (Salamanders go on a galaxy-spanning road trip to find and resurrect their primarch)
The Siege of Terra series has been pretty good so far also.