Books I wish you could buy in electronic form:
"A Magical Medley" from Grey Ghost Press. It's not too hard to find a copy of this 1997 paperback at a reasonable price even today (not least because you can still order it directly from fudgerpg.com), but a PDF would be a convenient addition. It's a collection of different magical systems and traditions with real variety and flavour, easy to use with any number of games but particularly intended for Steffan O'Sullivan's classic Fudge RPG.
These days I think it has largely sunk into undeserved obscurity; lack of a PDF certainly doesn't help in the modern market.