There are a ton of resources available online from Adobe for Lightroom if you look for them. Not to mention on YouTube.
Yes, indeed. I think
Kost's 2015 slideshow tutorial is a good introduction. It's presented in her breezy, businesslike "I-know-you're-in-a-hurry" style, but still manages to offer a comprehensive overview of the module's features. It would make sense, I think, for Adobe to assemble and maintain a usable index of the latest text and video resources describing Lightroom's functionality that it has produced. While they may all be searchable online, if you don't frame your query carefully you can wind up with an out-of-date explanation even though a newer one is available.
As a general aside, I personally have never found dedicated books on how to use a specific program to be very useful. If I want to quickly answer a focused question about a particular feature, they typically provide too much information to wade through in order to find just what I need to know, and present it too slowly and methodically. If I want to improve my craft, I find I'm better off with a book that emphasizes technique rather than the use of a single program's feature set, such as Jeff Schewe's
The Digital Negative.