Netherlands 1940 Wilhelmina Set

May someone explain these little mini-album sets? Were they bound into a book?

Not bound in a book (at least not to my knowledge) but they were simply approval pages back in the day when stamps were sent "on approval" to collectors. Stampex (the name in the LR corner) was an approval service dating back to the 1950s through (at least) the 1970s located in White Plains, NY. They heavily advertised in the classified section of magazines such as Boys' Life, Popular Mechanics, etc. Although there were exceptions, most of what they sold (as in the example shown above) were very common, low value stamps, typical of what a beginner collector would look for.

What does "on approval" mean?

Approvals are stamps that are sent to you to review and (hopefully) buy. You're supposed to select the stamps you want, send the rest back to the company along with payment for whatever stamps you retain.

It became quite popular 40 or 50 years ago and a good way to acquire stamps that weren't readily available locally. But it was plagued with problems. Stamp companies typically sent low value stamps at high prices. They started to market them to kids based on advertising in comic books, newspaper and magazines geared toward that age group. The incentive was offering a selected set of stamps for a few cents, then when the stamps came in, a number of other stamp selections were included "on approval", often for much more than their actual value.

The stamps the collector didn't want to buy had to be sent back promptly or they were billed for them. Sometimes they weren't sent back because of neglect to do it. Sometimes unscrupulous dealers claimed they never received the stamps back and billed the client for them anyway. In either case, invoices, interest and late fee charges started to add up, followed by threatening letters, etc.

Today, approvals are all but a fading memory, as auction sites and on-line sales have filled the void for most collectors.