Fair enough 
I will start with the DOE Handbook for Nuclear physics and reactor theory.
DOE HandbooksI am sure you know how to get to them but for those that need a little help the link above is provided to ensure you arrive at the right spot for said documents.
Oalsayari please don't stop there. You will need to have a big foundation in the fundamentals.
If you think that is really unnecessary then please let me give you a different impression. My company now forces all new initial licensee's to go through the System Operator (non-licensed) training to get the fundamentals and basic systems training before they attempt a license class.
You would be surprised (from what I am told) how much of it they just don't recall or have never been exposed too. These are smart individuals, such as, ChemE's, MechE's, ElectE's and etc. Time will tell if it improves our company pass rate. From my perspective it should greatly improve their chances of success.
As CM said, now is the best time to put in the efforts to get through all the important DOE Handbooks.
Pass it along to your future classmates.
Mac
Edit: Link was updated to reflect a change in address