I have played the Catalan for something like 25 years, and to me the big secret explaining the opening's popularity is the following: You can expect a massive score against weaker players.
The key strategic factors in a successful Catalan are:
1) You achieve control/dominance over all four central squares, something rarely seen in other openings. (By the way this explains why the Catalan set-up isn't so effective with a black bishop on
f5; white has too little control over
2) Long-term pressure against black's queenside, in part exerted by the bishop on the diagonal. In the end something is going to break.
These factors make it possible to turn up unprepared for a game Sunday morning during a weekend tournament and have a winning position before the first cup of coffee. In other words, great opening!
For books that focus on philosophy and not too much on concrete moves I can recommend the series How Wojo won.