I can confirm that the afternoon portion of each day is open response. I can tell you that is is absolutely required to have intimate familiarity with each of the four materials and connections between them. I do recommend using the Structural Engineering Review Manual (latest edition), but it will be necessary to find many more problems to complete.

The NCEES Practice Exam and Alan Williams' book "Structural Engineering Reference Manual" were by far the two most important items for me. I studied them both until I felt confident with every problem and page. I even found Williams' book to give me enough background to ace a few of the bridge questions when I didn't even own a copy of AASHTO. I would take them back to back, because it's a lot of preparation and stress to go through twice. And if you fail part you can retake just that part.

Oh, and see if you can incorporate it into your work somehow. Be creative; I wrote a spreadsheet to design masonry walls for example. I learned the details of how to do it by hand and I have the spreadsheet left for future use.

Good luck.

The test is all about time management and precision. SethGuthrie is right, get the NCEES Practice problems and exam. These are the exact format, difficulty level and style of the real questions. You should have a good method and mindset going into the exam. If you are a buildings specialist, I suggest the following.

1. Use one primary resource for each problem type, For concrete, steel, wood etc. use the code, or a textbook, or whatever resource, but learn it cover to cover. You can certainly bring other references, but you'll be better off knowing some books really well then having a bunch of books that you barely know what is inside of them.
2. The AASHTO manual is huge, you don't want to drag that thing out, then put it away, and then drag it out, etc. Instead, leave those 8 questions until the end and do them all in one go and then put that thing away.
3. Use some system of marking questions to return to later. My Method was to try and be done with 20 questions by the first hour. The idea is that about half the questions I should be able to read and immediately see what needs to be done and go directly towards the solution. Then in the next three hours I'm attacking the remaining questions I don't quite see the solution to, the AASHTO questions, and some questions which just require surfing through ASCE/IBC for some hyper specific block of text (maximum bolt spacing for ledger board connection blah blah sort of thing)
4. Bring a set of book ends so you can stand your books vertically, this really helps when compared to stacking the books in a random pile.
5. Buy or borrow the correct editions of the codes, you can always resell them for most of the cash back. I really don't think you want to go in with the wrong years code, because you really never know if that is the year they changed some Phi Factor from 0.80 to 0.75.
6. Be careful, the SE exam is much harder than the PE exam, I would say by 2 orders of magnitude. There are no easy answers, usually the difference between the 4 four answers are forgetting to apply the Phi factor, or forgetting to divide the a load by two if it is carried by two beams. It is very easy to wrongly arrive at the other three answers and think you've done the problem correctly.
7. Virtually everything comes directly from some code. If you ever find you are inventing a method, approximating a solution or coming up with some other idea of how to possibly solve the problem, don't even bother, because you are already wrong. One possible exception is that on the essay questions, with proper explanation, you may get away with something of this sort. One example, years ago my boss told me on his SE exam he had an essay question with a problem with a steel beam with a penetration in it. The question asked for him to compute the deflection. He didn't know an exact method, so he computed the I of the whole section, and the I of the section with the whole and used some value in between and explained his assumption clearly. I don't know precisely what score he got for that specific question, but he passed the test, so I didn't hurt him too much.
8. Take both sections in the same period, trying to separate your studying (into vertical and lateral) too much isn't a good idea (you'll end up with gaps in your knowledge) and if you just take one day, and fail that day, you'll really be dragging out the process considerably.

Good Luck!

M.S. Structural Engineering
Licensed Structural Engineer and Licensed Professional Engineer (Illinois)