After years of leaving money on the table for older D&D content and grumbling about all the Torrents of awesome homemade PDFs of their products (I know right!?) Wizards has finally release a ton of back catalog in PDF format.
Not only that, but instead of trying to do something digital on their own and failing, the partnered with a ready made solution and used DriveThruRPG to create the DnD Classics web site.
This new site has a ton of older manuals and adventures. Surprisingly it also holds a few 4E items as well (wow things become classics quickly these days!).
You should go there now and get the four free PDFs immediately. Why? Cause they are free!
B1: In Search of the Unknown (Basic)