It would appear that at least some of the Shatner Star Trek books are available as eBooks (at least in the US, you don't say where you are).
Links are to Amazon, but if they have them other vendors should too...
Spectre
Captain's Peril
Preserver
Dark Victory
Captain's Glory
Captain's BLood
Academy - Collision Course
I'd guess the others will become available over time as Pocket/S&S get to them depending on how fast they're doing backlist stuff.