It really depends on your familiarity with the subject matter. If the material is not something you work with or have seen before, you probably want at least two sources. I personally like to use the ASQ book, the CQI primer, and then purchase the CQI practice exams to test myself before taking the exam. This has worked well for me so far with the CQIA, CQI, and CQA exams.
If you are new to many of the concepts you may want to look at instructor led prep courses.