Wow your post really brings back memories, I still have three reel to reel recorders but sadly, at the moment, none of them are set up or in use. I remember making compilations such as the ones you are describing...and I remember commercial free radio too !
I didn't realise that you can't control the play order on the Kindle, that is disappointing, but if you selected your tracks carefully enough, it might not be problematic.
The tracks on the ambient sampler were chosen for maximum ambience, but it's entirely possible that some of them may still be too "active", and I think each person might have an ideal "mental" list of the best tracks to load up...
If it were me, I would probably have something like this
Eno - Thursday Afternoon
Eno - Discreet Music (the title track only)
Eno - Neroli
Fripp - A Blessing Of Tears
Right there, 3 or 4 hours of really beautiful ambient that I think would be suitable for a good read.
There will ALWAYS be the problem of some folk needing silence and others wanting music while they read - again, it's just differences/preferences but I do understand that if your partner needs silence, that could limit your ability to try out music while you are reading...I am lucky, my partner and I both love having music on while we read.
Even something like Fripp & Eno, Live, Paris 1975 is OK for us, it has some loud and dissonant parts, but the overall feel is so amazing, that we're OK reading to that...so I tend to be able to go even more active than your basic ambient style - up to a point.
If music is too busy, too active, or too loud - it is NOT suitable for reading...so as with everything, you have to find the right balance...
happy reading/happy listening