I've heard of Insel-Bücherei but until last week I haven't actually seen any. First published in Leipzig in 1912 their aim was mirrored by Penguin in 1935. That is, to bring affordable books to the common man. Their design obviously influenced the Penguin Poets and Scores although Insel-Bücherei's books were hardback, rather like the original Ladybird books, and the spine and cover labels were actually glued on labels.
There's been talk of them in The Penguin Collector but from before I was a PCS member so I may need to order back issues to find out more. A quick Google reveals the extent of Insel-Bücherai issues. I picked these up for £1 each. Not sure that'll be a common occurrence.
There's a digital collection here.