Just read and reviewed Xenos by Dan Abnett (click on the pic to get to my review).
Pretty interesting as far as these things go - especially after the first two Horus Heresy books turned out to be something of a slog (though I liked Fear to Tread, which I skipped right to after reading the second Horus Heresy book). Has a good mix of adventure, intrigue, and people and things getting blown up and/or dying in rather horrific ways. It’s the characters, though, that really stand out. I like most of them, and even the primary Big Bad is intriguing.
Just a note: if this is the very first time you’re reading anything out of the WH40K-verse, look up one of the wikis and keep it open in case something needs explaining.