welcome to the ILP community forum.
In my experience one of the best introductions into the ILP protocol stack is the one from the Boston Interledger Meetup. The slidedeck gives an overview of all the layers of ILP and its used protocols.
Regarding use cases, the HackHarvard in 2018 set out a price for the “Best Interledger Protocol API Hack”. The resulting projects provide some good inspiration, I think.
But it depends of course on how you want to use ILP. If you want to integrate ILP into your application, then the tutorials on moneyd, spsp, and web monetization might be a good starting point (also codius is a good example for the integration of ILP). If you want to run your own ILP connector as an operator, there is a good tutorial for that. And if you really want to deep dive into the protocols, I’d recommend the RFCs.
Disclaimer: I’m pretty new to ILP myself.