Regarding technical illustration, I can really recommend these books:
- Don’t Make me Think: This is a book about Web Design, but I think the principles apply to technical writing, too. As the title implies, you must not make the user think more than they have to.
- The O’Reilly Book on UI Design: This is really about UI Design more than technical illustration, but is useful if you are illustrating a software manual. This gives an overview of common UI elements and their recommended use. This can, at least, help to get your vocabulary correct.
- Micro-Interactions: This is again about UI Design, but gives a good overview of modern UI features at the detailed level. It also uses an entertaining writing style. There is one principle I want to highlight: “Use the Overlooked”. Items you may normally overlook can can actually be very useful. How does this apply to technical illustration? Take a flowchart, for example. The line thickness of the boxes could indicate the importance of a box, or make clear that several boxes belong to the same group.
I would also like to advocate learning basic Gestalt Theory. This is a description of how people perceive visually, and has many principles useful for technical illustration.