2001 Magnetics Design Handbook
A collection of articles by legendary master of magnetics, Lloyd Dixon. Highly technical. includes a good intro to magnetics, and can be helpful for anybody designing a switching power supply or the magnetics for a switching power supply.
2001 Magnetics Design Handbook - MAG100A
From there you can download individual chapters or click on the link "All 2001 Magnetics Design Handbook Sections in a single compressed zip file" to download it as one file.
Alternatively, you can download it as a single zip file by using this URL (working as of August 5, 2008)
If this link fails (they may re-arrange the site someday), go to www.ti.com and enter "MAG100A" in the search for key words box. That should get you to the page with the PDF files.