I wrote a several-paragraph reply that apparently disappeared, so in short: your sources are wildly mistaken.

There was rarely substantial album filler prior to Sgt Pepper. Prior to Rubber Soul, really, pop albums were just collections of singles; there was no reason to buy the album unless you loved all the singles, since you could just go buy the 45s to the songs you liked and screw the rest. A few artists like Sinatra crafted albums aiming for consistent mood and sound song-to-song, but it was by no means the norm. Songs were sold one-by-one, not in bulk.

The innovations of The Beatles and The Beach Boys led to album sales regularly outstripping singles sales for the first time in the mid-60s due to the album as product going from "collection of singles" to "full artistic statement". This change in how people consumed music allowed many artists to bundle more experimental quality work with pop singles knowing that the one would be purchased bundled with the other. Once the market shifted, you also had less consistent artists selling 10 pieces of drivel on the strength of one song that struck lightning, or of singles-driven artists producing filler in order to compete.

In the era of CDs this was especially pronounced in part due to pricing; if a single is $4 and the album is $12, and you already like 2-3 songs on the record...well, that record is gonna be sold whether the rest of it is good or not.

iTunes and other online pricing schemes have turned this around. Now that singles are priced proportionally to albums, people are less accepting of filler or of unfamiliar album tracks, and singles sales once again dominate the market - a trend we haven't seen since the 1960s.

Hope that's helpful.

I typically just lurk here and have no idea if this will be allowed as a top level post, but there will surely be people who know the answer to this question over on /r/Music

I would try there!

as I recall Pet Sounds came out while the Beatles were in the middle of production of Sgt Pepper. They were clearly very impressed by it and wanted to put something out comparable, but I dont recall them as being inspired to make that type of album because of Pet Sounds. I have one of the books with lots of quotes and studied that huge biography that came out in the middle of their career.

All or most of their albums always seemed to be filled with good music. Revolver, the previous album, is also considered by some to be just as good and really every song there stands out on its own. I dont think they consciously decided to make an album w/o filler since that is how they always operated anyways.

What exactly does your book say about all this?