I went on a Man from U.N.C.L.E binge and I've returned bearing recs.
Author: Evan Nicolas
Summary: Napoleon stood before him, arms crossed on his chest and a scowl on his face. "Nice exit," he said sarcastically.
This isn't slash but it does deal with homosexuality in a serious and sympathetic way. Illya and Napoleon each face deciding to support a fellow agent at the risk of their own careers. There is excellent characterization throughout. Should be a good read even if you've never seen the series.
Title: Grumpy Old Spies: Respect your Elders Affair
Author: E. Batagur
Summary: An illegal arms dealer learns about espionage the hard (and permanent) way. It's all about the Old School
I loved RED but it didn't occur to me until I read this to want to see something with the same attitude with other espionage characters who would also be getting up in years by now. What if Bond aged in real time? Emma Peel would still look great in a catsuit but only if John asked very nicely. In this case Illya and Napoleon can still blow things up and banter with the best of them. Slashy, but nothing R rated. You probably need to be familiar with the characters already to really enjoy this fully but a general feel for the genre is all you need to follow it.
There are more stories in this series that I haven't read yet here: http://www.soulsistahslash.
Summary: Napoleon is scared, Illya is hurt. The usual. Also, middle-aged men talk about sex. Food is mentioned.
Really excellent characterization makes this short speculation about how Napoleon and Illya might end up in a relationship as believable as canon to me. The author assumes the reader has a good knowledge of the show and doesn't need things spelled out. If you are new to the series and think you might be interested bookmark this for later.
Author Katya Baturinsky
An elderly Russian widow lives on the ground floor of Illya's building and spends her days watching over the neighborhood. As she gets to know her handsome young neighbor she speculates about his mysterious life. It's the glimpses of her own life and the respect that Illya treats her with that make this special and touching. Implied slash but only from the POV of a sentimental matchmaker.
Title: Safe Harbor
Author: Kellie Mathews
NC-17 Slash. Another how they get together fic. This one is much longer (75 pages) and follows Napoleon as he realizes his feelings for Illya aren't just friendship and then has to convince Illya. This is a bit of an AU in that it is established up front that Illya is gay and that this is known to his immediate superiors. I think that even though this story relies heavily on characterization they are clearly presented so that even those with only a vague knowledge of the show could just enjoy the romance (and lots of explicit sex scenes).
This story was a wild ride. There were bits where I cringed with contact embarrassment over some of the stupid things Napoleon does and I don't usually like stories like that but before I could pull out of the narrative something would happen and the mood would change. It's LOL funny, touching, sweet, hot, and a bit angsty by turns. The best thing is that the author expects the reader to keep up with both her plot and things like a scene where Waverly, a man with a classical education is totally confusing Napoleon with references that he lacks the background to follow and if the reader does too then it will be explained eventually, but if the reader gets it then the scene is all sorts of awesome.