Sun offers a style sheet for technical writers. These suggestions are used by Sun in its documents and are a very clean and internally consistent guide. Of course, it is not restricted to descriptions of Sun's products. A lot of care has gone into the guide.
Plus, the suggestions are not just for printed documents. Nowadays, a lot is expressed in languages with links, like HTML or PDF, and the advice pertains here too.
For illustrations, you are shown how to display interactable icons differently from non-interactable images. Subtleties, to be sure. But taking these into consideration makes for a clean document.
On the subject of documents with links, a chapter has cogent suggestions on the proper usage. Like avoiding overlinking, which is a common flaw amongst writers new to this. Also avoid linking with anchor text that is generic, like "click this". Instead, use more descriptive strings to help search engines classify the document that is pointed to. Assuming that both documents will be on the Web. But even if not, they might still be in your internal corporate web, and you might have an internal search engine spidering these. The strings will help the engine better classify both documents. This chapter may be the most vital of the book.