IMO, I really don't want to touch anything by Kyme.
I just recently finished Space Marine myself, and loved it. Lent it to another 40K fan, he's loving it too.
It's very lyrical, poetic, very 80's feel (duh, it was written around then). But Watson is a damn fine writer. Sort of akin to watching the Simpsons as they appeared on the Tracey Ullman show vs. their own incarnation (not that I'm a Simpsons fan, but you get my point).
I agree to go with Helsreach; it is masterful. The best Space Marine Battles book (although you should get Legion of the Damned), and a damn fine book on its' own. I recommend it to a lot of friends that don't touch W40K.
Storm of Iron, like Helsrach, is about a siege. Except in Storm of Iron, it is from the POV of Chaos. Graham handled it very well, this is a great book.
Graham writes the siege mechanics better, Helsreach is a more emotional book. It had strong resonations of Gemmell's Legend.