If you open many font files at once from the Finder, or preview a whole swath of already installed fonts in Font Book (by selecting them in the Font list and pressing Return), the Preview windows overlap, showing only the frontmost sample. To tile the windows so you can scan all the font samples, use Exposé’s All Windows feature (the default trigger is F9), and then click on one Preview window to bring it to the top of the pile. With Snow Leopard’s neater approach to Exposé (it arranges the miniature windows in neat grid rather than helter-skelter), it’s even easier to look through the font samples than it used to be.
When you have lots of Font Book Preview windows open for various fonts, either for a pre-install inspection or to compare various faces, it would be nice if you could use Font Book’s Close All command (by holding down Option as you open the File menu for the Close command) to close them. But Close All also closes Font Book’s main window, which, because Font Book is a one-window application, quits the program, too.
So, minimize Font Book’s main window (bring it to the front first, if necessary, with Window > Font Book). Once it’s safely in the Dock, Close All closes all the Preview windows without quitting Font Book, and then you can retrieve its main window from the Dock or with Window > Font Book.