Of course Sherlock would think that any gift John gave him would be inherently selfish. The man thought like that, cynical and cold, especially after the Adler affair (and John had never discovered quite how much of an affair that one had actually been, but when he tried to bring it up once, Sherlock had suddenly refused to speak to him for a week and had taken to playing adagios on the violin at 1:15 in the morning). Just because Sherlock had the reputation of being a reptile, it didn't mean the rest of the world was cold-blooded, though. There was such thing as a truly unselfish gift. So what could he get Sherlock?
A refrigerator? He had already tried it, and it hadn't passed the selfish test anyways (Just because he reaped some small benefit from it, didn't mean it was bloody selfish, just practical. He wasn't justifying this. They lived together. Something that benefited one of them inherently benefited the other. That was how it worked.)
New trench coat? Out of the question. Sherlock was so nitpicky about his clothing the man did the bloody laundry himself – and not in the usual London bachelor toss-it-in-the-laundromat-washing-machine-and-pray-to-whatever-gods-you-believe-in sort of way either. No, Sherlock actually did the laundry. In the kitchen sink. Every Tuesday.
(John had strong suspicions that the car that came to pick up the dressing gowns - the one article of clothing Sherlock possessed that he apparently could not clean himself - had, at one point or another, transported the British Queen. It would be very much Sherlock's style.)
Cigarettes? Mycroft would have him taken into a back alleyway and shot. Might actually do the shooting himself, come to think of it. Besides, Sherlock would smell it on him in an instant, and that would be that, Gift Number 5 down the drain.
Also, Sherlock was less of a pain when he was on tobacco. Went down from an infected cut to a sort of constant bee sting. Which made him easier to live with. Which benefited John. Which Sherlock would, of course, notice.
A book? Please. It was Sherlock. He'd have read it. Twice. And sent a furious note to the author pointing out any spelling errors.
Harpoon? No. Just no.
And the question remained. What could he get the most idiosyncratic genius who ever lived? Anything mildly ethical could be construed as selfish, anything not selfish wasn't even mildly ethical (John had considered, for a brief moment, getting Sherlock a case, then tossed it out on the grounds that he was not becoming the next Ripper just for Sherlock's wintertime amusements) Other than an ten-foot-monument and a throne to control the entire universe – which he probably still wouldn't be grateful for – John could not figure out anything that Sherlock would like. He didn't seem to like anything, besides experiments and Ms. Hudson and the Adler woman and very rarely John himself. He certainly hated people. Hated…oh.
There was the gift.
John looked around for the nearest copy shop and, smiling wanly, ducked in.