In a little recorded piece of publishing history Jonathan Cape in 1934 issued a series of small books called Flexibles. They were cloth covered books with dust jackets but the covers were much thinner than hardbacks and flexible. They were a sort of half-way house between paperbacks and hardbacks. The first Penguin paperbacks appeared the next year and may have caused the premature demise of this series after only 10 books. They were quite stylishly presented and pleasant to handle. All were reprints.
The first in the series Lewis Browne's The Story of the Jews was probably re-issued as a counter to the rise of Hitler. Others in the series include Hemingway's Men Without Women (uncommon now especially in the jacket) Joyce's Portrait of the Artist and later Dubliners, followed by Beverley Nichols Twenty-Five. The last 'flexible' was Italian Backgrounds by Edith Wharton, number ten in the series. All came out in 1934 and as far as can be ascertained there was no number eleven. Amazon has this review of the fifth book in the series Sarah Orne Jewett's The Country of the Pointed Firs:
No plot devices or car chases here--this is a book to read on a rainy afternoon when nostalgia and melancholy threaten to overwhelm. It's comfort food like grandma used to make--reassuring, soul-fortifying, and full of the capacity to cheer. It's also addictive--once you take a bite out of Pointed Firs, you can't stop.