Some of the Youtube stuff is pretty great--Skills With Phil, Seth's Bike Hacks, GMBN, etc. Mastering Mountain Bike Skills is a pretty good book if that's more your jam.

I'd recommend mostly riding a lot and focusing on basics--don't worry too much about all the skill drills at first. With particular regards to descending, focus initially on picking a good line (look up some videos on cornering technique) and maintaining a good attack position (look this up--basically, focus on getting your butt of the saddle and keeping your weight relatively balanced between the wheels, which will be more forward on flats, and more rearwards on descents. Consider scribing your seatpost at the correct height for pedaling so you can easily lower it for better handling on descents and locate it back up for climbs. I would advise against upgrading much on your bike except consider getting a dropper post if you're really enjoying the bike and want to improve its descending prowess.

Brand bike manuals are useless, particularly on a hardtail like yours (they can have some useful information for full suspension bikes, although this isn't usually in a manual but rather on a technical document or an exploded diagram). They mostly are a bunch of CYA legal safety disclaimers. If you want to look up a manufacturer document on any component they're usually relatively easy to find online.