What are good study guides/practice tests for the airframe maintenance certificate?
I used the Dale Crane series of study guides from ASA years ago, available from either of those two links as well as many other places.

Here is the specific study guide for Airframe, but you will also want the General study guide. These are for written test prep, but might also include Oral and Practical (O&amp;P) test prep.

  

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There are also other options out there. Gleim offers an AMT test prep course, as does ASA in their AMT Prepware, but I don't know that I have any experience with either one. I definitely used one of the digital test prep packages out there, but I can't remember which one. I do like the digital tests, especially since that is probably how you will be taking the writtens anyway.

I also studied the Jeppesen textbooks in addition to the FAA textbooks you mentioned.

You will also want to study the Practical Test Standards (PTS) for your O&amp;P test prep.