Okay, the title of this page is a bit facetious. But it's also serious. A lot of people think fanfic is amateur in the worst sense of the word: lacking in skill compared to professional fiction. I disagree. Of course there's bad fanfic out there. There's also bad profic. Ninety percent of everything is crud, in the immortal words of Theodore Sturgeon. But fanfic isn't failed profic. It's a genre of its own. If judged by the rules of conventional pro fiction genres, yes, it falls short. But pro fiction, judged by fanfic standards, also falls short.
The books on this list really are almost as good as fanfic. They've got a lot of the qualities that I love about fanfic. Specifically, an emphasis on characterization, relationships, and angst, to the point that you might call it "emotional porn." There's also "subtext." The strongest relationship is between two male characters - what you might call a homoerotic element.
The genre is somewhat related to Jacqueline Lichtenberg's "intimate adventure." Not quite, however. For one thing, for me, the adventure part is optional. For another, her definition lacks the homoerotic element. "Slashy" probably comes closer, but I didn't like that term, either, because many fans use that term to describe any gay fiction, and gay lit is not at all the same thing. Also, slash often brings to mind something explicit, not subtextual. "Almost As Good As Fanfic" was the best description I could come up with.
Without further ado - my list of pro books that are almost as good as fanfic:
The Heritage of Hastur, by Marion Zimmer Bradley
Exile's Gate, by C.J. Cherryh
Faery In Shadow, by C.J. Cherryh
The Foreigner Series, by C.J. Cherryh 
Hunter of Worlds, by C.J. Cherryh
The Tale of the Five, by Diane Duane (The Door Into Fire, The Door Into Shadow, The Door Into Sunset, The Door Into Starlight [forthcoming])
The Coldfire Trilogy, by C.S. Friedman
(Black Sun Rising, When True Night Falls, Crown of Shadows)
Kill the Dead, by Tanith Lee
House of Zeor, by Jacqueline Lichtenberg
Unto Zeor, Forever! by Jacqueline Lichtenberg
Related Links:
Viridian's list of Some Slashy Books That Leap to Mind
Katy's list of slashy pro books
Slashable Novels - Kathara's slashy novel recommendations. Includes links to other slashy book recommendation sites, as well as to a Yahoogroup devoted to the topic.
The Slash Novel Review Site - Pro novels rated and reviewed according to their slash appeal, complete with summaries and warnings of elements that some readers might find upsetting.
Romentics.com - A site that sells gay romances (male/male). They expected to market them to gay men, but to their bemusement, most of their customers are straight women. Imagine that...