There is a very good book that explains everything you will need in embedded systems using Linux. I has a complete chapter on RTC.
Linux Device Drivers Development by John Madieu, published by Packt Publishing 2017.
This book doesn’t cover BBB directly, but it covers all Linux subsystems, such as SPI, I2C, RTC, DMA, etc
If you are looking for a book that covers BBB and the build cycle and root file system, then this book is very helpful:
Mastering Embedded Linux Programming - Second Edition by Chris Simmons, published by Packt Publishing 2017.
These two books will get you up and running so you are able to help yourself. I wish I had these books 10 years ago. These are the only two books that deal with the latest kernels and Device Tree.