A simple Google search for "AP Exam Study Guide" should bring up more than enough material to choose from; you can narrow the search based on the subject(s) you need to study for. Here, for example, is a website with lots of information about specific topics and areas.
One option is to use the AP Achiever (Advanced Placement Exam Preparation Guide) fo European History authored by Chris Freiler. It has practice tests, covers a review of trends, and includes document-based answers. It also has timed practice test with answers and sample essays. It incorporates released exams in the study material. The College Board highly recommends using released exams.
Barron's is good, and so is Cliff's, but I also like the "Cracking the AP..." books. They have a more casual tone to them, more like the writer is talking to you, the student, and they have a lot of strategies and hints about how to approach the various tasks on the AP exam. I would also second the suggestion above and use your library! They will likely have a lot of these kinds of test-prep books. Ultimately, the best source of information about the test is your teacher -- be sure you are picking his or her brain and asking about what strategies you should be using to study for the test and to be prepared for the actual format and expectations of the exam.
I have used Barron's guides before in my own teaching and tutoring with students and I can definitely second the recommendation. They are particularly good, in my opinion, at helping students themselves to realise the difference between a grade 4 and a grade 5 response, and what you need to do to bridge the gap.
Your best bet might also be to ask your teacher about ordering copies of old exams. I actually never really liked the study guides, as the questions were invariably more fact-based than anything on the exam. But Barron's is probably the best of the bunch. AP European exams actually don't deal that much with cause and effect in the essay portion- they tend to deal more with sweeping social and intellectual trends and change over time.
I used to use a book called REA, but I don't know if they have them anymore. I also like Cliff's notes. I have heard, though, that 5 Steps to a 5 is the best. The key is accurate practice tests. Don't forget to ask your teacher if he or she has suggestions, study guides, or study sessions or practice exams.
Find out who the publisher of the test is and then see if they also supply a study guide for it. For example, if your test publisher is Pearson, then search for that study guide. Also, this may seem archaic, but it is also economical, go to your local library and see if they have past study guide books for that same test by the same publisher. You could compare and contrast many different tests from the past and see what has changed from year to year. Just make sure that you note any changes in requirements or format so you don't study the wrong types of questions. Most exams for History, however, focus on cause and effect scenarios; so pay particular attention to events that cause other events. (Such as the Serbian who assassinated the Prime Minister of Austria that started World War I and revealed secret alliances across Europe and the world.)
My favorite is Barron's. I really like them for all history AP exams. What I like best about them is that they have explanations about test-taking strategies and they incorporate those into their explanations of the answers to their practice tests. So they'll do things like telling you "here's how you should have been able to narrow this MC question down a bit..." Between that and the fact that they have 2 sample tests in them, that's my favorite.
I would recommend that you not take both tests right away. Maybe take one to figure out what areas you're weak in and then save the other to test yourself again after you've done more review.