The test is much tougher than it used to be. I took it last October, and the failure rate was 49%. I passed with an 89, but I studied the guide for three months. You should NOT rely only on the study guide. The test purposely asks you questions found NOWHERE in the guide. I have a near-photographic memory when it comes to multiple choice tests, so right after I walked out of the test I wrote the questions down that I felt I missed. I checked them in the guide and some were not covered anywhere. I found ten of the questions I thought I had missed, and they were, indeed, wrong. When I got my score back there were twelve more questions wrong and I had no idea what they were. The trouble is, ISA won't tell you what you missed and they won't give you the right answers to ones you know you got wrong. It's very frustrating and makes the test a non-learning experience. That is, if you are doing things wrong on your gig, and you answered a given question wrong based on how you do things, you'll still keep doing whatever it is you do, wrong. I went a round with ISA on this and they wouldn't budge. They're still a great organization, but I wish they'd give feedback on wrong answers.
Study other tree books. Read up in more depth on pruning, construction around trees, etc. Some of the questions on the test are poorly worded, no doubt about it, but what can you do? I guess the pink slips let you vent, afterwards, but big deal. Some of the ID pictures are a joke. They are not clear and some leave out defining characteristics, altogether. Get the tree list for your State from ISA and then grab the pics off the net. I made a great file of every tree on the list and also collected 90% of the tree leaves, flowers and seeds right around my town. It helped.
Good luck!