The Sennheiser PXC250 is a good match for your needs. In my opinion it's a substantially better-sounding headphone than the PX200, and the active noise reduction, while not all that strong, is plenty for most library situations -- it'll make things like air conditioner sounds and nearby whispering go away just about completely. I've been seeing them for about $70 refurbished lately, which is a great price.
You might also want to consider the Sony EX51, EX70 or EX71 canal phones. They're inexpensive, super comfortable, super portable, and block a decent amount of outside sound. On Head-Fi it's fashionable to sneer at these, but I really enjoy mine. I own ER-4's, E2C's, HD25's, various other cans and an array of portable amps, and my EX71's still have a place in my bag.