I am! Sorry I forgot about this area!
The first Warhammer book I ever actually picked up was False Gods, the second book in the Horus Heresy series, I really dont think its a series that can all be read as stand alone novels. I understood so little of it that I gave up on it until I found the first book almost a year later. In the mean time I ate up the Ghosts, I am a rabid fan of them, Dans work actually, is perhaps the best of the Black Libraries authors, which isnt to say the others are bad. I have read up to book 6 and stalled out to read the newest Ghost book, and got caught up in The Dark Tower. I hope to soon get back into them. I personally found Fulgrim and Flight of the Eisenstein to be my favorites. Watching Fulgrims descent into the heedless folds of Slaneesh was perhaps one of the greatest things Ive ever read, while I rooted from the Death Guard (FotE) to keep to their ways and not break.
PS: My grandfather also loves these series, he also has a soft spot for the Ultramarines. He enjoyed the first two Heresy novels that he is working on a picture of Loken and Torgaddon during the Oath of Moment ceremony.
I HIGHLY suggest picking up a copy of Double Eagle by Mr. Abnett. Its a great air battle filled book. If you have made it to Guns of Tanith you will recognize a few names as the main squadron is from the planet contested in GoT.