Passed Water Resources in NY. I found the water breadth materials by PPI exam prep to be lacking with a lot of errors. I used just the civil book during the exam. I won't lie I did get one or two answers from there, but I think I could have gotten there with the civil book too.
The best studying I did was practicing looking up where to find the formulas or methods in my reference books. Might be common knowledge, but they are useless unless you know where to look.
When I went through the "answer key" to a problem I did not get right the first time, I made sure to highlight where to find helpful info in a reference book, tab it and come back to do the same problem multiple times over a few weeks until I could look up the useful concepts or formulas in seconds based on how I tabbed or highlighted.
My studying was more training myself to know where to find what I need.
If all else fails during the test, solve by balancing units! If you have to guess, make educated ones. Sometimes you can eliminate an answer or two that way. Like if you are asked about a factor of safety for a discipline you never use, and something less than 1 is a choice, probably not the answer (it could be I guess but you would expect something greater than 1).
Not sure it helped me or not, but I passed, first try with about 30 hours of studying.