If you are seriously pursuing precancels, the best thing you can do for yourself is to join the Precancel Stamp Society. Their monthly journal often has articles about who used which precancel and when. Once you are a member, you can access their digital library.
A couple of catalogs which should be purchased include the Town & Type catalog and any catalog for an area you want to specialize in. There are literally millions of precancels, when you take into consideration the use of a precancel type on each different definitive stamp (and sometimes on commemoratives).