Study materials can be a mixed bag
The ASE offers study guides for your test. In my mind, these are analogous to a rubric. These are the definitive breakdown of what is on the test and what percentage of a test a given topic comprises.
So far as I am aware, the ASE does not publish actual materials for you to study, considering it a potential conflict of interest. Most techs choose one of two ways to prepare. One is not using study guides at all. That’s certainly a valid way of approaching the situation, but I personally wouldn’t do well like that. The other popular scenario is using study guides provided by third-party companies.
Books put out by independent third parties are foundational and don’t always keep pace with the test evolution. Sadly, many are not updated often enough. I’ve used test prep materials from some larger companies and come away from them realizing they’re very helpful, but not a magic bullet. I’ve seen some books where older, less relevant material isn’t pared down enough on the next edition, so you wind up unwittingly spending your time studying antique car parts. Multiple books, be they guides, OEM materials, or cast-off vo-tech stuff, are good arrows to have in your quiver but staying up-to-date is the most difficult part of study, I think.
When you feel like you’re ready to test your knowledge, the ASE is probably your best bet because they offer practice exams and presumably their questions will be a pretty good representation of material covered on the test. If you bought study materials, odds are excellent they will also have practice tests. The downside here is after you tally fees for the tests, the study materials, and the practice tests, costs start to really balloon. You’re gonna have to balance what you know against how much you can spend.
On an editorial note, I would like to mention that every employer I’ve ever asked for help with test fees and study materials has chipped in. Maybe I have great luck, but I think they were interested in helping me get certified because my certificate in the frame in the waiting room helped them, too. Remember, it’s free to ask your boss for assistance.
On the practical side of things, nothing beats shop time, but you should have that in spades already. Make use of that situation. Bring stumpers into the shop with ya to work through with fellow techs. A colleague may be able to explain confusing concepts in a way you can better understand. They can also help ferret out errors in practice material questions, which are way more common in most of the independent study guides than you might expect. Finding errors is valid study, too.