When I first started in oncology, I purchases Oncology Nursing which is published by Mosby. This book is my top recommendation as it includes everthing you need to know to care for oncology patients. I personally learned so much from this book! It does include OCN prep questions in the form of case studies, which I think helped me pass the exam.
There is a new publication from the Oncology Nursing Society, Cancer Basics. It is a good, easy-to-read book with very basic information. I think it geared more towards someone just entering the speciality that wants a good foundation, but it is important to know that it does not go into much detail.
Cancer Nursing by Connie Yarbro is supposed to be a very good read, but I have never gotten around to purchasing it. Many co-workers like to use it and have nothing but good things to say about it.
The core curriculum book is OK. I did not like the format (outline style). The study guide had a lot of great practice questions for the OCN.