Ultimate Power is the sourcebook that expands the rules for superpowers found in Chapter 5 of the Mutants and Masterminds core rulebook. Essentially, it's an expansion (or replacement, depending on your point of view) of that chapter.
Normally, sourcebooks that are nothing but rules bore me to tears. I like sourcebooks that have stuff on characters, settings, themes, etc., stuff that can fire up my imagination. Stuff that's nothing but lists of rules, spells, equipment, what have you, is about as fun to read as a cookbook. Combined that with the question that if all this stuff was so great, why wasn't it in the main book? Bottom line, a book like Ultimate Power has to be very good for me to consider this anything more than an occasional necessity, the RPG equivalent of toothpaste. Luckily, this book delivers in spades.
Chapter 1 is about the practical concerns of handling superpowers in general. Things like the range of powers, how to counter them, how to notice them, and other factors are all delved into. It's all very handy.
