You'll need to know all the information in the study guides.
Most people with a good experience in welding will need to put effort into the mechanical testing of welds, discontinuities, and NDT. Don't overlook welding safety.
Your experience reading drawings will prove to be useful in reading welding symbols and understanding the welding processes you are already familar with. Spend some time learning more about those processes you are not familar with.
Pick a welding code you are familar with for the open book examintion. If you use AWS D1.1 or D1.5 everyday, why not study it for the exmination. Likewise, if you use API 1104, then by all means, use it for your open book examination.
Good luck - Al