Very nice series of books that I look forward to rereading some day. The frequent description of 'Harry Potter for Adults' made me fear it would be totally derivative, but it really wasn't in any way, shape or form that bothered me.
I felt the the first three books worked better than the last one. I normally don't care for mysteries in books, especially not in 'romance', since they are usually stupid, far-fetched, simplistic and yawn inducing. And they function as filler for me at the most (Sorry, Mr Lanyon). In these, we don't get the mysteries shoved down our throats and lo and behold, that low key approach worked much better for me. And it was never who I thought it was, even when I refused to believe it was the obvious person. It all added up on a more subtle level than expected. The last book doesn't really have a mystery and that turned out to be one of its weak points. While I truly appreciate the absence of a Sauron/Voldemort/Hitler type enemy, the 'solution' relied too heavily on the powers of the main character being larger than life and the obvious need for Frost to embrace his female side. Maybe this one should have been written from Frost's POV, but then again, the angst may have gotten debilitating in that case. It was hard enough to understand why Kyle was even in love with him as it is with all the snarling going on. It got a little preachy too.
So. The sex. There's lots of it. Especially in book 2, when Kyle studies the 'Esotheric Arts' for a while. Not all of it explicit and none is kinky, but it is there in such abundance that I should point out that the reader needs to be okay with m/f, m/m, m/f/f and m/m/f/f or else this is going to be flung at some point.