If I had the time (or if I could read as fast as my 10-year-old daughter) I would definitely want to read:
Dr. Euler's Fabulous Formula: Cures Many Mathematical Ills
by Paul Nahin
Considering the huge pile of books waiting to be read (and a few of them to be reviewed as well!), I've only read the dedication, which is one of the nicest I've ever seen:
"For Patricia Ann, who (like Euler's formula) is both complex and beautiful."
aaaaah. somebody found the formula for love there