I'd start here
http://freeasestudyguides.com/.
There are also online tests that you have to pay for in advance. Not overly expensive.
Being north of the border I'm not ASE certified. I am licenced by the government. Certification here is compulsory, so I had to attend a formal college apprenticeship course. Three terms of what they call accelerated intake; one year of information in just two months; or three years in six months. After receiving a certificate of apprenticeship I continued to log hours until I was eligible to challenge the exam. There were some study guides at the time, but long out of print now I believe. You should consider yourself lucky that you can do online studies now; an option that I did not have.