And if it works for Baen, there's no telling what other publishers might try.
Baen also issued a bunch of back-catalog books on CDs, bundled with various hardbacks. If you missed out, you can get all the CDs for free as downloads or torrents:
http://baencd.thefifthimperium.com/
Torrents are listed at the bottom of that page.
These are legal, Baen-authorized downloads (the CDs may be freely shared), and include some of their more-recent titles. (I don't know how good the torrent is, but the 5th Imperium site is reliable -- slow, but steady).
A few years ago I bought a hardback Honor Harrington book, included was a CD with ten or more previous books. A couple by David Weber and I think David Drake. If I can find it I may go back and read one. Weber's fantasy series now sounds interesting. I need to see if the formatting is compatible with my Nook.
I think Baen has been doing E-books since there was such a thing. Not sure how long they have been giving away free e-books though but it has been quite a number of years.