See what books your program has. Most peds resident lounges have a ton of reference books and things like that quite handy to look things up.
Many people like the Harriet Lane, but personally I can't stand it. Poorly organized and has some mistakes. Likewise I don't much like epocrates. Their peds dosing can sometimes be a little off. Lexicomp is a great program, but costs money, as is Up to date in terms of educational stuff. You'll also get a subscription to Pediatrics in Review, which is a great educational resource and worth reading through when you have any time.
So overall, I wouldn't rush out to buy anything in particular, but see what's available during orientation. They will give you a Harriet Lane, maybe a Redbook and a number of other useful resources that you'll need as an intern.