Everyone needs the most current Harriet Lane. Its the definitive reference for lab values/vital signs/drug doses/whatever. Odds are your program will give you one but otherwise you need to buy it.
I like baby 411 and toddler 411. They're Q&A books written for new parents. Spend a weekend on each one, reading it cover to cover, and you'll sound way more competent when you hit clinic.
For NICU I like Polin and Yoder's practical Neonatology. Its basically a case files book for NICU. Hard read, but doable in a 4 week NICU block if you push. If you finish it you'll sound like a superstar.
Finally there is Zitelli. Its a great reference, and you'll probably buy it for board review. If you have some spare $$ I would buy it now, though it ccan definitely wait if you are short on funds.