Finished The First Heretic... idk it really just felt too fragmented throughout. The Custodes plotline wrapped up well like you said it would though, TSR. I also enjoyed the kind of fraternal / genetic rivalry angle with them that Lorgar made very explicit. Maybe one day we'll get a book just about them, they've always been so mysterious in the universe and I'd like just a dumb Space Marine Battles type thing about them.
Speaking of shit I've read though,
all of these are free to a good home:
Fire Caste claimed
The Emperor's Finest claimed
The First Heretic claimed
As always, I only ask that you (1) be in the U.S. or have me ship to a U.S. address and (2) swear an Oath of Moment to never profiteer off of Black Library's low print runs in the future with any of these books if they ever get valuable in the secondary market. (I think Fire Caste might someday if Peter Fehervari ever makes it as an A lister Black Library writer.) Also I ask that you please be a little patient with delivery, I do go to the USPS a lot in my line of work, but it's not every day.
If you want my mini-takes on all these books:
Fire Caste - Probably the most unique 40K book / game / audio drama I've consumed but it is kind of insulting to your intelligence with a few unsubtle pop culture references. It's also not an easy read, not in the sense that literature isn't an easy read, but in the sense that it doesn't hold your hand with the important revelations in the story and you really need to piece the heavy shit together yourself, and no, it's not one of those books where the author has stayed off record about what the ending really is. If you read these books on the train or when you're otherwise occupied / able to be distracted, this won't be the book for you. If you're interested in the Tau, especially if you wonder if "everything that glitters isn't always gold" with them as they are probably the least abhorrent currently known civilization in 40K (not that they aren't abhorrent, they just are the least abhorrent), it will give you a different perspective on them, and especially their caste leadership and philosophies. It is kind of an Imperial Guard book in parts, but there's also an examination of commissars as a twisted stereotype, and commissars as people asked to carry incredibly heavy emotional burdens so I think that's worth the Zzz of the Guard stuff.
The Emperor's Finest - My first Cain book. It was pleasurable enough, I probably don't want to read another one for a long time because I can see how the common criticism of them (that they're repetitive) is very true. It's got civil wars, Astartes, the Mechanicus, Tyranids, Orks, and Space Hulks so you're getting a lot of cheap 40K excitement in a little book. I was familiar enough with Cain from his first audio drama to kind of fill in the stuff long-time fans know about, but if you know nothing about Cain you might want to at least read a Lexicanum article about him before tackling this one.
The First Heretic - I don't feel like it was ADB's strongest work and it's fragmented in bad ways (long jumps of time, weak emotional and intellectual characterization). I wanted a Lorgar biopic from it and it's definitely NOT that, but if you want to get a taste of how the legion who worshiped the Emperor as a literal god became the mendicant order of the Ruinous Powers you'll get that in here. It'll also maybe make you reconsider some of the fundamental things we take for granted about the mythos, but as always remember nothing you read in a 40K is canon!