I don't know the whole story behind it, but it seems you got illegal and legal realbooks..
The transcriptions in The Real Book are unlicensed, meaning that no royalties are paid to the artists whose songs appear in the book. Consequently, the book violates copyright and is therefore illegal. In the past, it was usually sold surreptitiously in local music stores, often hidden behind the counter for customers who asked. PDF editions of the book are now often available on P2P networks.
I like the idea of giving all people the opportunity to learn those old jazz songs, so I think they should be legal. They are in my opinion and so I don't feel guilty downloading them and learning those songs. I actually like the romantics behind it....
Btw there seems to be software for it, for Mac and Windows...
EDIT: btw: Improvisor seems to include also different jazz and blues songs like 'autumn leaves' and 'tears in heaven'. http://www.cs.hmc.edu/~keller/jazz/impr ... Index.html
I'm now learning to play 'Summertime'
Maybe (in future) different linux musicians can learn the song on different instruments and then record each part or play via netjack2....