Yellow-back is a type of book that was popular in the late 19th and early 20th centuries. These books were typically cheaply made, with flimsy paper covers that were often yellow in color. The stories inside were usually adventure or romance novels, and they were targeted at a mass-market audience.
Think of a yellow-back book like a fast food hamburger. It's not the highest quality, and it's not going to win any awards for its nutritional value, but it's cheap, easy to get, and it satisfies a certain hunger. People who wanted to read for fun without spending a lot of money could buy a yellow-back book and get lost in an exciting story.
Yellow-back books were an important part of the publishing industry at the time, and they helped to open up reading to a wider audience. However, they were also criticized for being lowbrow and lacking in literary value. Despite this, many people still enjoy reading yellow-back books today as a way to escape into a different world for a little while.