Best thing, if you accepted the book but realize within a few steps (maybe immediately) that you didn't actually want it, would be to walk back to the person handing it out and say "Changed my mind, don't actually want it, why don't you give it to someone else?" I know some people who hand out religious tracts or other such materials, and every one of them that I know personally would accept the item back with good grace. They'd rather give it to someone who will actually read it.

And if they're the kind of person who won't take it back with good grace? Place it on the ground right next to them, and walk away. Make it their responsibility to deal with it. (If you don't want to go out of your way to find a trash can: some public spaces make them easy to find, but others not so much).