I don't carry a book as a reference. What exactly do you think you'll need to be looking up at the bedside? In my opinion, it would not give your patients (or their families) much confidence if they saw you looking up their child's diagnosis while you were in the room.
Your unit should have pediatric drug references, which you should absolutely refer to for every medication with which you are unfamiliar (outside the room). They also may have critical care references on hand if you have a question about something. You'll likely also be able to access the internet to look up specific things. Again, outside the patient's room, before you care for them.
Pediatric critical care is one of the most diverse fields of nursing. You'll see patients ranging from newborns to young adults, from 2 pounds to 250(+) pounds. You'll see respiratory, renal, cardiac, neurological, and surgical diagnoses. There simply isn't a "pocket" guide that can contain all of that information.
You might be able to find a "fast facts" guide that includes normal vital signs, common medications, pedi weight calculations, and drug calculations, PALS info... But this will by no means be an all-inclusive guide.