Emma, the best known combo is tomatoes and basil. Grow them together and each deter the other’s pests.
Here’s a link for companion planting:
http://www.bbc.co.uk/gardening/basics/techniques/organic_companionplanting.shtml
For witchy stuff you need
Encyclopaedia of Magical Herbs by Scott Cunningham. – a fascinating book from an American witch who sadly died a few years ago, but he wrote lots of good books on basic witchcraft stuff.
If you want gardening with a difference, try
Heritage Vegetables by Sue Stickland (1-85675-033-7)
Dee