I got my copy of Angels of Darkness for the princely sum of £6.99 straight from a GW store about 10 months back- also if your going to read 'Descent of Angels' you have to read 'Angels of Darkness' before or straight after it.
I thought it was the worst of the Horus Heresy series so far (mainly because its set about 2-3 centuries before the Heresy) but then I was in a GW store the next day and the guy there told me that some of the characters are in Angel of Darkness, and that its full of references that you'd only get if you read the Angels of Darkness (wish i'd known that before

)
EDIT: HOLY CRAP! theres a copy going for $278 on Amazon!