Most stereoview production ceased in the 1920s, however Keystone continued to make cards for schools (mostly elementary) and libraries well into the 1960s.
Because these were used in libraries by sticky fingered children, and may have been poorly stored, they are most often in poor to very poor condition.
Most novice dealers will over state the condition of an antique photograph and there is no 'standard' as in stamp or coin collection.
But condition of the individual cards is key. NO fading. NO solarizing (gas on water look in dark areas or near edges),. Photos firmly mounted to cards. NO scratches, blotches, marks or rubbing on photos. Firm, un worn edges on the cards.
These cards sometimes used negatives that were 100 years old, so the image content can vary greatly. Of course collectors prefer original issues of images, some images will be favored over others. I am not familiar with the actual numbering and title of this set. Sometimes sets were created for a certain catalog (like 1935) and than changed, or even discontinued the next year. Also sets could be made to order or even altered by the library's own order requirements (for instance a library in Chicago may want to have more cards on the beef industry, while the library in Louisiana would substitute cards for the shrimping industry -even for catalog listed sets. But most often these sets were sold as listed in the Keystone catalog.
All of the cards should be numbered. I believe the two volume set should have 100 cards. They may or may not follow sequencial numbering, and if this was originally a 1000 card set, cards could have been mixed up between sets. Also, if they were sold by the library some time ago, subsequent owners may have tossed cards and added cards to the collection in the box, that didn't belong in the original set. So, numbering is key to collectors.
A general guide is that these cards, as a set in 'good' condition, may fetch $400-$600 if complete for that set. Of the original set was quite large, like 1200 cards (yours was at least 550 or 50 per volume), then there would be cards of great interest to specific collectors, as mentioned above rare photos of certain industries, which can bring a good value per card (like $10-$20 to the right collector). Other cards, like the "Chicago Stock yards" were included, for some reason, in just about every set that Keystone ever made, and may not even be worth the price to ship them.
In most cases a lot like yours will be bought by a dealer who will break up and try to sell individual cards.
Condition, again is everything, below are some price guides for very good condition. For good condition, half the price, for poor condition, donate them to your local charity or sell as a bulk lot.
Images of notable interest:
East: China, Japan, India, Russia -especially old city views and cultural industries ($5-$8ea)
Modern transportation: Blimps, Hot air balloons, Plains, Trains, Automobiles (as in show cars, like "the fastest land record..." not just parked on the street) ($10-$60)
Monuments; Statue of Liberty, Mount Rushmore, Pyramids, Eifel tower, etc. ($5-$25)
Ethnic interest: as individuals these may sell better in sets of 3-12, like sets of Norway, or France, Java, etc. ($1-$3 ea, in sets)
Anything that you look at and go "Wow" (like riveter working on a 50 story tower above New York in 1925) will probably be of great interest to collectors too.
Anything that you go, "that's weird, I wonder if..." like the mom milking a goat -probably not that interesting.
Cards like these seem to have lost a lot of value in the past year or so. I am seeing individual cards, like of a president, that would have sold for $25-$50 eight years ago, hardly show interest at $5 now.
