I took (and passed) this last year. I used two books:
Neonatal Certification Review For The CCRN And RNC High-Risk Examination
Certification and Core Review for Neonatal Intensive Care Nursing, 4e
First of all, sorry for the huge font; that's what happened when I copy/pasted from Amazon
A couple of notes, with the first book, it comes with a code where you can take online exams which give you an idea of your progress though I really felt some of the questions/answers were bizarre and odd. Just use your critical thinking and discount those if you notice. For example; a woman presents in labor with high blood pressure, headaches but no lab evidence of PIH/HELLP (good liver enzymes, no proteinuria); you should monitor the infant for CVA because mom's obviously on cocaine!
The second book is the question/answer only book, not the core curriculum book a lot of people use. All the questions have rationales of not only why the right answer is right but why each of the wrong answers is not right. I found this book incredibly helpful. I could not get the online access to work on it.
I helped my husband prepare for and pass the RNC just a couple of days ago using these (and a seminar his hospital held). I would offer these up but I've already promised them to coworkers who are scheduled for the test.