Edward G. Hoffman’s Jig and Fixture Design is a great introductory text on the principles and practice of designing jigs and fixtures for parts machining. While oriented towards the machine shop, the book is handy for those making other sorts of fixtures for woodworking, welding, etc. While you rarely need a jig or fixture for making a single item, when making multiples of a given part, a jig or fixture often reduces the amount of setup time and ensures consistency from part to part.
The book is insanely expensive new, but you can find used copies of the current edition for a manageable sum, and earlier editions are often available for next to nothing. I paid $5.50 for my copy of the third edition at a local book shop.