Applied Akka Patterns by Michael Nash, Wade Waldron (Oreilly) was very digestible and relevant at the time, might be dated by now. Just read the intro to get the vibe.

These days I would recommend picking a framework and then ask claude & friends to do a deep dive with you and build an example project out. Ask it to explain concepts, architecture, trade-offs, scalability considerations, hosting considerations, compare it with other frameworks, hook it up to storage systems (sqlite, postgresql, blob storage) and so on. Try running them within a wireguard network and so on. Very interesting learning to be found.