Hi Alex, as far as I know there are no in-print books that really fit your description regarding theory. That said, the Winston/Keith book is probably your best bet, there are some useful theory sections and a good mixture of beginner and intermediate exercises.
https://www.amazon.co.uk/Pedal-Steel-Guitar-Winnie-Winston/dp/082560169X/ref=sr_1_1?ie=UTF8&qid=1535542207&sr=8-1&keywords=pedal+steel+guitar+winstonA quick reference for chords might be up your street, not sure if you can still get it but Buddy Emmons "E9 Chord Vocabulary" might be helpful, it's just a two-sided card. Or Jeff Newman's Chord Dictionary goes into more depth.
Here's a really useful video for guitarists moving to E9 PSG:
This is a great website with tons of theory
https://steelguitaracademy.com/Your big challenge is going to be physical: how to get everything coordinated, in tune, with good tone. Use anything that inspires you to sit down and practice, whether that's playing along to records, being in a band, Skype lessons, watching youtube, getting in on jams etc... up to you. I would include some general tab/song books in that picture just for variety. Plus learning to read PSG tab is going to be an important skill in it's own right.
Best of luck with it,
Jonathan