While there has been considerable debate in the geek microcosm of the Mac community over whether repairing permissions in OS X does any good or not, what hasn't been debated is that understanding permissions can empower an OS X user and lift them into superuser status.
Knowing how to use Terminal or the Finder's Get Info dialog can give the superuser greater control over how files behave. Want to delete that stubborn file out of trash? Want to know how to share your iTunes library with more than one user? Want to customize your Apache installation? These are the types of situations I use my knowledge of permissions on.
If permissions are something you still think you need to get from your mother, or you don't know why you would want to understand something so geeky, check out Take Control's new book, Take Control of Permissions in Leopard. This 87 page ebook will answer basic questions from "Why do so many problem-solving sites suggest that I repair permissions?" to more advanced questions like "Now that the NetInfo database is gone in Leopard, how do I edit account settings such as numeric UID?" Given that any discussions of permissions in OS X is geeky to an extreme, you might be relieved to find out that this book recommends utilities you can use to manipulate permissions without getting into the dirty, unix underbelly of Leopard.
However, if you like getting dirty, there's plenty of unixy goodness to be found its pages. This ebook may cover how to take control of permissions via the Finder, with Mac utilities, and using the command line, but it also covers heady topics such as access control lists, setting up and manipulating account groups, and editing account settings, all via the command line.
All that and only a one megabyte download.