Author Joe Kissell has been informing and entertaining Mac users for years through his books, TidBITS columns and Macworld articles. He's recently finished a pair of books for the Take Control ebook series that will help you learn how to keep your Mac running at its most efficient pace, as well as tell you what to do when something does go wrong.
In the first book, "Take Control of Maintaining Your Mac, Second Edition" (US$10), Joe describes actions you can take weekly, monthly, and annually to enjoy top performance from your Mac. He also tells you what not to do, illustrating tasks that take a lot of time and provide few benefits.
The second book is an update to Kissell's "Take Control of Troubleshooting Your Mac" (US$10). Joe's expertise really shines in this ebook as he tells you what to do with Macs that won't turn on, how to react when you're getting repeated kernel panics, and actions to perform when internet connections go down or printers are balking.
If you already own an earlier edition of either one of the books, you can get a discount on the upgrade. New buyers can bundle the two books for just $16 for a top-notch set of guides.
In the interest of full disclosure, TUAW blogger Steve Sande has written several titles for the Take Control series.