"The Decca Portable Gramophone" - Christopher Proudfoot - CLPGS Reference Series (No.35) - First Edition 2010
Produced in 2015 this is a 24 page illustrated account of the Decca portables available from 194 to 1956. Despite its small size this is a must have booklet for any collector of Decca machines.
"The Columbia Phonograph Companion Volume 2" - Robert W. Baumbach - Mulholland Press - Digital Edition 2014
As with the George Frow book, this is expensive to buy in hard copy but a digital version is now available from iTunes at more affordable price. It deals with the American Columbias and therefore has limited use in the UK. Interesting none the less for its historical account of the American Graphophone Company.