In my opinion, the way Manning does this is pretty close to ideal. If you buy a physical copy of, say The Art of Unit Testing[1], or C# in Depth[2], either directly from them, Amazon, a bookstore, whatever, you get a code to download the ebook as well. Especially because it is usually only about $10 more expensive to buy the physical book + ebook combo, rather than just the ebook from them.

[1] http://amzn.to/29Rchjc

[2] http://amzn.to/29LASq7