I bought a copy of The Smithsonian Book of Mars (2008) by J.M. Boyce yesterday and find it a great addition to my space scence library. I'm normally hesitant to buy planetary science books for a general audience because they tend toward the coffee-table variety with lots of fabulous pictures but little in-depth science. This book seems to bridge the gap nicely: It still has some cool pictures, and the science is relatively up-to-date, comprehensive, and not too dry to be readable. Format-wise, it resembles some of the books in the Cambridge Space Science series, but for the general public. I haven't read the whole book, but thumbing through it has left me with a very positive impression.
I'd be interested in knowing what others at UMSF think.