Frostburn has been released as an ebook -- a secure form of PDF. It's currently available for sale through DriveThruRPG.

Having a D&D supplement in PDF format offers a number of advantages over paper books: The text is bookmarked and searchable, making it easy to find information in the middle of the game. An ebook doesn't weigh anything, allowing you to carry a large collection of books on your laptop without lugging a 30-pound bookbag to the gaming table. (Of course, currently only Frostburn is available as a PDF--but other D&D titles might be in the future.) And, in limited amounts, you can cut and paste text from your Frostburn ebook -- if, for example, you want to paste a monster stat block into the adventure you're preparing, or copy the text of a feat onto your character sheet.

The Frostburn ebook is a secure (or DRM) PDF. To purchase it, you'll have to register your copy of Adobe Reader 6.0. (Go to Tools-->eBook Web Services-->Adobe DRM Activator in Adobe Reader.) Once you've done that, the Frostburn ebook is just like any other PDF -- you can open it on other computers you own (as long as the copy of Adobe Reader on that computer is also registered to you), print all or parts of it, make copies of the file, and so on.