I despise AP's but that's just a taste issue, the first DM I ever played with was horrible for railroading us without them, so I played with a new DM and he decided to use an AP; it was the first campaign he ever ran, after the first railroad my character became a nuisance, I found every reason not to follow the railroad eventually I died and didn't roll up a new character. I get why they exist but I sandbox homebrew all my games, I let the characters decide who they talk to and from that I have little mini-adventures for them to go on from interacting with an NPC, and over time I build up a large adventure that they either discover was related to a bunch of the mini adventures or a large event happens and they can rise to the challenge. I don't have a difficult time coming up with interesting NPC's most NPC's I create are just character's that I would love to interact with and have thought of, aside from that I have a copy of each of my characters I've ever played at each of their level's and I photocopy the player sheets from my players when they level up, I have eight years of campaigns (my group was running four campaigns at one point, we had a lot of free time and played every night and all day weekends), so I just recycle old hero's who are either still adventuring or have retired and taken up a new trade, sometimes I will reskin them with different abilities or races and almost always use a different name. BAM! Instant awesome NPC.
The only books I care about when they publish are things like The Advanced Players Guide from Pathfinder, the core books, monster manuals, I have a bunch of stuff from older D&D that I convert, like Aurora's Whole Realms Catalogue from FR, the more of that stuff they publish the happier I am.