This model is posing for a photo session in an artist's studio. She takes her inspiration for the pose from a figure drawing book. It looks like it is Andrew Loomis' Figure Drawing for All It's Worth.
The open page in the first photo looks like page 194, which illustrates distributed weight in a figure.
The nearest page in the second photo looks like a combination of 2 illustrations, page 102 and 106, which illustrate twisting and turning of the figure.
Andrew Loomis was born in 1892. After school he moved to Chicago, and worked in editorial and advertising for most of the top clients at the time including Kellogg's, Coca Cola, Lucky Strike and more. He published Fun With A Pencil in 1939 and his hugely influential Figure Drawing for All It's Worth in 1943.
Figure Drawing for All It's Worth 2011 edition
Figure Drawing for All It's Worth (How to draw and paint) 1971 edition
Fun With A Pencil