Can someone please take the time to explain Book Moves in a little more detail?
Back in the day it meant the move was recommended in a book somewhere. It means the move has history. More loosely it means more than 1 strong GM has played the move more than once in a long time control tournament game against a serious (GM) opponent. In other words a few people who are very good players and who really studied it decided the move has good points (whether objective, practical, or a mix of the two). Now, 50 years later people might discover the move isn't so good for some reason, but that's basically the meaning of the word book... it means it's a reasonable move with history behind it.
Are these Book Moves based on known openings?
Yes. All known openings are book moves... with the exception that most of the very dubious or obscure lines probably wouldn't be called "book." It's basically a combination of quality and history... if either of those two terms are zero then it's not book.
Are Book Moves a good thing?
In general yes. If you're playing a lot of book moves it means your opening was somewhere in the range of sensible to high quality.
Should I be trying to increase the amount of Book Moves I play?
Not necessarily. Look at why you're losing the game. If you're getting into untenable positions in the first 10 moves, then yes, you need to be playing more book moves...
But the opening principles allow you to play book move in most positions without memorizing anything. So try that first. Also, sometimes it's not possible to play a book move (see the next question below).
Would a really good player have 20 book moves in one game?
If their opponent cooperated with them, then yes, that's definitely possible.
Remember most openings (other than systems) are a set of moves by both players. So if I play a horrible move on move 5, then I've taken the game itself out of book. After that neither of us can play book moves anymore.
Is there a number I should be shooting for?
Not of book moves, no, but I recommend memorizing the first 5 or so moves (5 moves by both players) of the main line of any opening you play. This means when you get to move 3 and there are 5 different "book" moves (i.e. popular moves) then pick the most popular one (the one with the most number of games in the database) and memorize that.
So lets say you play 1.e4 so you might memorize 5 moves deep in the main line of the Scotch, 5 moves in the main line of the Sicilian, 5 in the French, 5 in the Caro, 5 in the Pirc. IMO doing that plus knowing the opening principles gives you a good start.
If you play 1.d4 with 2.c4 then maybe something like: Queen's gambit accepted, Queen's gambit declined, Slav, Nimzo indian defense, king's indian defense.
Should I be trying to get more book moves in my game?