The vast majority of 20th century books and pamphlets will be printed via offset press or as letterpress. What’s the difference? Offset transfer the ink to a rubber blanket before its put to a surface. It provides a more even print on less even papers. Letterpress directly applies the plate to paper which also produces a slight indent around the lines. It can make for slightly higher contrast in illustrations due to the faint indent.
Then there’s the tiny fraction of other things that were done using other duplication processes. Many items that were produced in very small batches for special events , local groups, etc were produced using various other methods that may require special handling. If you handle lots of ephemera, you may encounter alternate printing processes. Many fanzines, chapbooks, pamphlets, and APAs may use this methods. Also, if there’s no date on the item, but you CAN ID the process used, it will give you a better idea of the date and how many copies were probably produced.
