I took the TX Ag Engr. PE exam in October 2004. I was lucky in that I had some older classmates and business associates who had taken the exam in the last 2-3 years. Their material and study guides were fairly current.
As TerraSouth said, dig out all of your old text books. The Midwest Plan Service articles were helpful, especially the publication #1 (MWPS-1), which is very broad in its topics of coverage. Many old work out problems referenced guidelines and industry standards listed in the MWPS publications. MWPS-18 and MWPS-29 were also helpful. You'll want to have a good reference for agricultural electricity and wiring standards. I found Agricultural Wiring Handbook by National Food and Energy Council helpful. A copy of ASAE standards was helpful.
You'll want to be familiar with the psychrometric charts. I sent off to ASHRAE for good quality charts in many different units. During the exam you are provided with charts, but I recommend using the same charts you studied with during the exam so you are familiar with what you are using.
I created a couple of binders where I summarized information, or at least organized pertinent standards, methods, specific constants and factors which may not be in a specific book or manual. In these binders I also kept examples of problems with much detail as to equations used, where constants or safety factors were found, and steps to layout problem. In my exam I was able to use these example to help layout real problems during the exam.
Greatest of all, I allowed a good 3 months preparation time and created a study schedule so I was assured to at least refresh on all topics. Some topics where I did not have work experience or college classwork, I only studied briefly, conceding I probably would not be able to answer many questions in these areas. I aimed to be really proficient in areas where I had work/college experience.
I worked many practice problems too. I had practice examples from when the test was not only multiple choice, but there was a workout/written test section too. I think working these workout problems was very helpful even though current test is all multiple choice.
I've gotten long winded, so I'll bid you good luck too. When do you take the test?