What If? is the eponymous book of the web series by the author of XKCD. If you want to know whether you'd like it, read the web version (I like the web version). There is some paper-only material, to give some impetus to buying the dead tree version. It's good material, along the same lines.
What's my absolute favourite bit of it? The bit where it says "To Simon" at the front. I attended a book signing by Randall Munroe, after he gave a talk at the Royal Institution. He talked about the "physically building the periodic table" what-if, and the logistics of building a giant ball pit. It was great. My second favourite is the "bullet as dense as a neutron start" entry, which is extremely good.
As you might have guessed, highly recommended.