I am learning from John F. Collins' A Primer of Ecclesiastical Latin, since I am most interested in Church Latin and more than one person recommended it to me. There is an answer key for the drills and exercises.

I typically alternate between doing a unit in Collins and trying to decode some piece of 'real' Church Latin that I use daily (currently Little Office of the Blessed Virgin Mary, Baronius Press) with the assistance of Collins, on-line dictionaries, and (when I get really stuck) latin.stackexchange.com.