I actually think you can have two. In the rules it says, with bolded words included, "Each student may bring one commercially published book (or field guide)". I guess you + your partner = two students, so if each student can bring one you can have two total.
2011~Dynamic (1st Regional, 1st State, 36th Nats) ~Birds (1st, 2nd, 39th) ~Remote (1st, 3rd, 42nd) ~Wind Power (1st, 4th, x)
2010~Dynamic (1st Regional, 1st State) ~Egg-O (x, 6th) ~Birds (4th, 5th) ~Remote (1st, 1st)
2009~Egg-O (11th State, 36th Nats) ~Herp (6th, 44th) ~Remote (x, 36th)