I've been making very slow progress through the books I got for Christmas, but I did finish a good one this past weekend, and I thought I'd recommend it. It's Mr. Timothy, by Louis Bayard, and it imagines the life of Tiny Tim from A Christmas Carol.
Tim is a little older and a little wiser...and not so very well off as you might expect. He is, in point of fact, living in a brothel, where he earns his keep by teaching the mistress of the house how to read. From his vantage point (both a part of London's grimy underworld and apart from it) he spies what he thinks is evidence of something more criminal than the consensual activities that go on under his own roof.
For Dickens fans, the book is a gem, touching on all the themes so dear to his heart--literacy, the complex social strata of society and the possibility of movement between them (both upward and downward mobility, by a variety of means), and the plight of the most vulnerable members of society. London, one of Dickens's roundest characters, is well-represented in Bayard's book as well. But even if you've never actually read A Christmas Carol or a single page of Dickens, the thriller is sharp enough to woo a mystery buff or two, and the characters are thoroughly engaging (the requisite ragamuffins are particularly fun, and there's a sea captain who might make you rethink your retirement plans...especially these days).