Paper Books Bonibooks
The books contained in the BookScans Website provide a study of "mass market" paperbacks ... a concept that is generally considered to have begun in 1939 (at least, in the U.S.) with the first numbered Pocket Book editions. However, long before that, Albert and Charles Boni began implementing their dream of providing affordable books to the masses.
Albert Boni left B&L Books (Albert Boni and Horace Liveright) after only a year, in 1919, following some sort of altercation. B&L would eventually become Random House. (Liveright, for his part, was a huge influence in American literature, and it would be well worth your while to look him up online. He was a very early advocate of First Amendment rights in publishing, and spent months in court arguing against John Sumner for the right to print Petronius's Satyricon -- almost 2,000 years after its first publication.)
Albert Boni's paperbound "Little Leather Library" is covered elsewhere in the "Oddities" Section. That began in 1920. However, the size of those books made them more a novelty than a serious publication.
Charles Boni began printing paperback books by himself in 1929. His first books were called "Charles Boni Paper Books," and they featured artwork (often very elaborate) by Rockwell Kent. The illustrations wrapped around the outside of the digest-sized books, and adorned the endpapers inside both front and back. They sold by subscription at $5 a year for a book per month, or in stores for 50¢ each.
After the first year, however, something changed. The "Paper Books" became "Bonibooks," and the title pages (while keeping the same colophon) changed to include both brothers, as seen in the scans below.
In his write-up for Bonibooks, Graham Holroyd notes that the last ten "Paper Books" (numbers 6 - 15 in the printing order) also had hardcover printings done using the same artwork (none of the books were actually numbered). Graham also lists the publisher as B&L, but I cannot see any evidence of that from studying the books themselves.
There were more than 60 books in the run, all between 1929 and 1931. Several were re-issues of previous titles. In mid-1930, some of these paperbacks were issued in hard cardboard slipcases. The cases bore the same artwork as the books themselves, like the last picture below. The books in slipcases are exceedingly rare and fetch a hefty price today.
(Click any scan to enlarge.)
The image of the book & slipcase is courtesy of Between the Covers-Rare Books, Inc.