The Witch (a new class for "Basic era" roleplaying games) by Tim Brannan (author of Ghosts of Albion) is now available in print from DrivethruRPG.
The 120-page book is written primarily for the Labyrinth Lord system, but is easily adaptable to other retro-clones and systems designed to emulate that particular style and era of roleplaying.
Published by Jason Vey's Elf Lair Games, the softback edition will set you back £15.50 - but you can get the PDF free with that if you want (it's normally £3.10 on its own).
It covers the "Witch" character class and the various traditions of witchcraft one might encounter in an old-school fantasy game. This sourcebook contains plenty of information for players and game masters alike, including new spells, powers, magic items, and monsters.
Bear in mind that Tim is the "go-to" guy for all witchy matters as they relate to roleplaying games - his other writing credits include all the Buffy The Vampire Slayer Cinematic Unisystem books, including The Magic Box - and his Willow & Tara campaigns are legendary in certain corners of the Interwebz. So he knows what he's writing about!
Check out his blog - The Other Side - for an "Appendix N" of his research materials.
I've got a copy loaded into my Drivethru shopping basket and will pick it after Christmas - once my finances have regenerated slightly from their seasonal hit.