This weekend I re-read the Original Dungeons and Dragons (OD&D) rules from 1974 - you know, the white box with 3 little brown books by Gygax and Arneson - and I noticed that it says explicitly that clerics use spell books just like magic users.
This was an amazing revelation to me! One of my favorite aspects of D&D is the spell book mechanic derived from the 50s-era fantastic fiction of Jack Vance. Having magic users quest for scrolls, books, and mentors in order to expand their magical repertoire is always a major, if not central, character motivation in the games I run.
I relish the thought of applying this mechanism to clerics as well. Searching the planet for lost prayers, meditations, inscriptions, and holy books in hopes of gaining the ability to invoke favors from ancient, and possibly forgotten, supernatural beings would add a lot of fun and mystery to a campaign. I also think it makes things much less technical and gamey than every cleric simply picking the same old spells from the same old list before every game session. Certain orders or cults would of course have their favorite holy books with selected canonical spells and prayers. What good cleric wouldn't want to bring back new invocations for their order to add to their canon?
As far as I can tell, the clerical spell book first was discarded in the "Blue Box" Basic Set by J. Eric Holmes. I am disappointed that none of the editions of OD&D retroclone "Swords & Wizardry" use traditional OD&D clerical spell books.
I hereby declare that, starting today, all clerics traveling through my games must learn their prayers and invocations from mystical scripture.