Howdy and welcome!
While Programming in Scala isn’t bad, it’s not actually the book I tend to point beginners at: it’s a little large, and sometimes more detailed than you care about. The book I’ve been starting to use for new folks recently is Essential Scala, which seems to be working well – it’s a nicely structured course on the topic, with both teaching and exercises, and the students I’m facilitating seem to be enjoying it.
(And regardless of which sources you use, please don’t hesitate to ask questions here.)
Ecosystem is harder, simply because it is almost overwhelmingly large. The Scala library ecosystem itself at least has Scaladex, a searchable index of the available open-source libraries, but that doesn’t touch the vastly larger number of Java ones. I’m not sure if the Java side of things has anything as straightforward to use. (Not to mention all the other languages in the JVM ecosystem that can potentially interoperate.) I will admit that I’ve tended to go via word-of-mouth there.
(Although, in practice, I tend to find Scala wrappers on Scaladex rather than use raw Java libraries anyway – while they interoperate, the idioms are rather different, so you tend to benefit from having an adapter that translates a Java library into friendlier Scala idiom.)