From the 1940s to the 1990s, home encyclopedia sets such as Encyclopedia Americana (Grolier), Encyclopedia Britannica, Collier's Encyclopedia, and World Book were sold exclusively by publisher's commissioned salesmen. Other books were typically sold in retail stores, via direct mail, or via subscription services such as Book of the Month, but encyclopedia sets simply were not. If you asked about an encyclopedia set at a bookstore, you'd be told they were not carried, and possibly the bookstore would ask for your address so an encyclopedia salesman could visit. (They were getting a referral fee from the publisher.) Likewise, if you tried to order directly from the publisher, you'd be told that because of the complexity of encyclopedia sales and the various options, policy required that an encyclopedia consultant visit your home. These publishers also offered products such as the Great Book of the Western World collection you mentioned. It was the publishers' choice not to use these sales channels.

Initially, the salesmen worked door-to-door. Unlike many salesmen who targeted housewives, encyclopedia salesmen wanted to visit when the whole family was home, so tended to work evenings and nights. Bear in mind the cost of an encyclopedia set during this period typically ranged from $300 for a bare-bones set to $1,200 or more with luxury bindings and add-ons like "great works" collections, annual updates, and other reference volumes. So these were major purchases which few households could afford with their cash on hand. The salesman's commission represented perhaps a quarter of the price.

The encyclopedia salesmen acquired a bad reputation because of their deceptive, high-pressure techniques. (This is why the originator of Great Books of the Western World had feared them becoming involved.) Critics commented that encyclopedia sets were not bought--they were sold. The core of the sales pitch was to present the encyclopedia set as an educational investment that was necessary to the household, particularly its children, and that could be afforded for "pennies a day" through installment plans (i.e., credit purchases) that were not always clearly explained. Salesmen would basically try to guilt-trip parents into purchasing encyclopedia sets and then point out that the second car or smoking habit or after-work beer cost much more, so how could they justify depriving their children?

Salesmen sometimes lied to gain entry to the home, for example by claiming that they were not selling anything but were instead gathering opinion data or deceptively informing the prospects that they'd won a contest or been selected to receive a free encyclopedia set for test-marketing purposes. In the test-marketing scam, the prospects would be told they were among a handful of households in their community selected to receive a free encyclopedia set, in return for which they would be asked to write a testimonial letter and provide feedback. The encyclopedia would be improved in response to their feedback and updates would be sent. Of course, there would be some costs for the updates, just "pennies a day." In fact, they were simply being sold the encyclopedia set at the customary price, with the fib about a free encyclopedia being a way to get through the door.

There is a 1969 Monty Python sketch in which a woman suspects a well-dressed visitor of being an encyclopedia salesman and refuses to let him into her home until he promises that he is merely a burglar come to ransack the home. Once he enters, he validates her fears by showing no interest in burglary but rather trying to convince her to buy an encyclopedia set. An announcer then cut in to identify this as a successful sales technique, which he contrasted with the unsuccessful sales technique depicted as the salesman being thrown out an upper-story window. This should give you some idea of the disrepute into which encyclopedia sales had fallen.

In the United States, classic door-to-door encyclopedia sales declined in prominence in the 1970s because of changes in consumer protection law. Most notably, in 1972, the Federal Trade Commission required that most in-home sales include a three-day "cooling off" period during which the buyer was free to rescind the transaction, return the merchandise, and receive a full refund. Under threat of further regulation, around 1976 the big encyclopedia publishers instructed their salesmen to stop making classic "cold call" door-to-door visits and instead follow up on prospects. These prospects were developed through media and direct-mail advertising campaigns, which the encyclopedia publishers had always run. So this was the dominant mode of home encyclopedia set sales in the 1980s-90s: someone sent in a card or made a call to ask about buying an encyclopedia set, and the publisher sent a salesman to close the deal. Of course, the in-home sales pitch's basics were the same, and now the prospect had already expressed interest.

Salesmen themselves had high turnover. Some felt bad about the techniques they were trained to use. Some didn't like having doors slammed on them. Some complained they'd believed they were applying for marketing or research jobs rather than commission-dependent direct sales. On the other hand, the high turnover and insistence on "college men" for the job made it easy for investigative reporters to get hired, go through training, work for a day or two, then file exposes.

What really killed off the home encyclopedia set was the pre-internet home computer of the early 1990s. Legacy encyclopedia publishers had realized it was just a matter of time before reference works were digitized and replaced their sets of twenty-plus bound volumes. Initially they came out with computer versions of their own encyclopedias packaged on CD-ROMs and including some multimedia elements such as audio and video clips. They priced these multimedia encyclopedias at about what their bound products cost, in the $500-900 range. Software giant Microsoft undercut them with its Encarta product, a multimedia encyclopedia that eventually retailed for about $99 and (worse for competitors) was often bundled with new home computer purchases so seemed "free." Developments past that start running into the 20-year rule, but basically with greater penetration of home internet and the realization that updates can be pushed more rapidly, you see an attempt to maintain revenue-generating online encyclopedias as subscription services before that model was undercut by free online services such as Wikipedia.