When my sister was pregnant with My Liege (aka my nephew CZ), I bought her a present every month on her due date to count down. A lot of them were anthology books: Pooh, Paddington, Beatrix Potter; things daddy could read aloud and 'talk' to the baby without simply talking too the belly. Now that he's 5, he is enjoying reading them himself... and reading to his sister (age 3).
The most special book I bought (and have bought for several other little ones in my life) is The Princess Who Grew Down... which was my favorite book from childhood. Thank goodness for Albris.com, which allows me to find copies whenever I need one... even though it is out of print.