Boston, I don't know much about the ISA test, but I'm a Certified Massachusetts Arborist, so I know about the MA cert exam.
I agree with Guy about Harris's book, but Pirone's is good too about general tree maintenance. The third part of Pirone's book, Abnormalities of Specific Trees, is good but forget about the "controls" of tree pests as the info is outdated and the MA exam won't cover controls anyway.
Make sure you have all the current ANSI A300 Standards for Tree Care Operations, as well as the ANSI Z133.1 standards, and know them well because much emphasis will be placed on them. For some reason, I guess because there's not a lot of math in arboriculture, certain arbo exam designers seem to revel in torturing innocent arbo examinees with a couple or three math questions, and these questions invariably surface in fertilizer questions. So it's a good idea is to get down your fertilizer math. Know your fertilizer analyses and ratios to a tee, and be able to translate them to rates of application for trees.
For the MA Cert oral part of the exam, there's no way you can prepare for it because you never know what they're going to ask you, so just good, thorough general prep is all you can do.
Tree ID is big in the MA cert exam. You probably are good at this already, but it will entail a slide show by (I think still) Jack Ahern, head of the Landscape Architecture Dept. at UMASS, Amherst, also a tree-climbing arborist. It's a slide show, and since I've taken courses by MadJack Ahern, I know his repetoire of slides, so if you're interested I can send you a list of plant materials that he'll draw upon in his slides.
And of course Treeco's suggestion of going to an arboretum is excellent, but I'm sure you've been already to the top arboretum in the country a few times!
Good luck!