Here are two sites to check out. The first is hands down the best online resource for the PICU, consisting of an entire PICU course from the Society of Critical Care Medicine. Find out what types of cases you will see in the PICU at the hospital and focus on those areas. There are a couple of PICU books but there really is no "best book" for peds ICU.
for basic stuff, i would read up on your physiology. acid/base, blood gases, ventilation, frank-starling type stuff. then i'd brush up on your ID (lots of spetic-ish and empiric abx in the PICU) and stuff you don't normally use in regular peds, like vasoactive meds and their indications.
PICU is fun if you go into it with the right attitude. be pro-active about procedures-- they're fun
--your friendly neighborhood currently on PICU himself caveman