Then there are large PDFs where somebody ran the old paper fake books through a scanner, but they are just images and are not in an actual computer readable format, so they can't be transposed.
So the computer has not solved the notation problem, yet.
Myself, I've memorized most of the standard jazz repertoire.
In fact I suspect that most of the written music repertoire that exists today will never be translated into computer readable form, because it's just too much work. And not enough new music is being composed to form a critical mass around some new notation system or computer format. When somebody composes a tune (I play in one band that does original jazz compositions), they send out a PDF.
Maybe software will eventually automate the process reliably enough to be useful.
Also, a lot of the written stuff is handwritten, not typeset. So it's a subset of the handwriting recognition problem.
It's pretty bad for the bassist, since the bass part is usually the second to last part to be copied, meaning that the copyist was probably drunk. ;-)
I don't know...ebooks still look a lot like treebooks.
The one example I've seen of what you're describing is I've seen jazz musicians play off ipads. This give them access to very large catalogs without lugging around giant binders, and also they can transpose their sheets into any key.
(That's more important than it might sound at first, because different instruments "play" in different keys -- if a pianist thinks the piece is in C major, the clarinetist thinks the piece is in Bb major. So now you don't have to have separate books for the different instruments in the band.)
But the music as written down in these electronic fakebooks is a lot less complex than your average classical piece, so it's a much more tractable problem.