Without saying any detail I can tell you that having references beyond the CERM saved me at least 4 questions on the morning exam....
I think the key ones are an enviro/civil dictionary, a basic geotech textbook, a basic transportation eng textbook that has many of the tables from the Green book etc..., and while I did not have it I really could have used some scheduling info text (i.e. project management material beyond what was in the CERM). I had a surveying book but never had to use it.
I took water in the pm and I had Wastewater Engineering, Gupta's hydro systems, as well as other water related books but only ended up using those two and the dicitionaries I had along with the CERM.
Everyones background is a little different. I can tell you in the PM I did approximately 2/3 of the problems without even opening any text simply from studying and in the morning that number was close to 1/2. I would not probably of passed with no references (closed book). I probably would have been passing but too close for comfort with just the CERM and passed with a cushion having the additional references.