Here are a few great examples of books illustrated (many also written) by Lois Lenski (1893 – 1974)
Erik Nitsche (September 7, 1908 - November 14, 1998) was a Swiss-born designer who, after moving to the United States at the age of 26, had a long and successful career as a graphic designer and art director. He's most famous for the work he did for General Dynamics, the engineering company, but also created many iconic record album and book covers. I love his abstract geometric images humanized by delicate color palettes. Enjoy!
I only just came across this illustrator the other day. She's been working hard in the industry for many years and is happily still at it! Her work is available on Imagekind. Here are some especially nice book covers and a catalogue cover from the 1950s. I love her line work in these book cover illustrations, and the catalogue gives us a taste of her quintessentially mid-century style of character illustration. She also created some whimsical advertisements while working in Chicago, New York and Boston. See more here.