As I am not too experienced with Java (8 months) I might have overprepared, but here is what I did.
For the 4 months I read daily between 30-50 pages of one of the following 3 books (Boyarsky and Selikoff more than once).
1. Boyarsky and Selikoff - OCP: Oracle Certified Professional Java SE 8 Programmer II Study Guide: Exam 1Z0-809
2. Sierra - OCP Java SE 8 Programmer II Exam Guide (Exam 1Z0-809) 7. Auflage, Kindle Ausgabe
3. Ganehs, Kumar, Sharma - Oracle Certified Professional Java SE 8 Programmer Exam 1Z0-809: A Comprehensive guide.
The first book – is a must read, the good thing about Sybex is that they only give you as much as you need to know, no less but also no more, and that is important when you have so much material. It also helps to know where the boundaries are.
Second book is a great addition as it explains many concepts in better detail. Boyarsky and Selikoff tend to keep things short and to the point. This book is 1400 pages though, pretty much double as big as the previous one, so a bigger time investment.
I also highly recommend Java 8 in Action, as it was a great help with Lambdas and Streams.
Enthuware: Great investment (only $10 or so), I tried to do daily tests, just don’t start with them early as you will run out of the questions you haven’t answered in a month. So would be good idea to save it for the last month.
Practice tests from Boyarsky and Selikoff: great resource, especially cause questions are harderto memorise than on the Enthuware website, so you can keep redoing them.
Happy with the result, in case you want to read more about my preparation tips, I wrote a blog post about it as well:
Blog Post: How I passed OCP