My recommendation of the day: the City Walks card packs published by Chronicle Books. This isn't a travel guidebook in the traditional sense, but instead it's a box of cards, which I adore for two reasons:
First of all, they're insanely easy to pack and carry. You won't find yourself weighted down with a bulky book when you usually end up using only a small portion of it. You can bring only the walks that appeal to you and save loads of valuable packing space.
Secondly, for being so efficiently sized, they're quite thorough in their information. One side of each card is a detailed walking map, the other side is a description of where to go and the significance of what your seeing.
And these walks aren't typical, touristy and boring, either. We'll be using the London version this May. And considering this will be my sixth visit to London, you can imagine that I get pretty excited when I find walks I haven't been on before. Needless to say, these walks don't disappoint.
City Walks are available for a myriad of cities worldwide. Check 'em out!