van Gulik, R.H. (1956) Siddham: An Essay on the History of Sanskrit Studies in China and Japan. New Delhi: Aditya (reprinted 2001).It's part of Śata-piṭaka series produced by Lokesh Chandra and his illustrious father, Raghu Vira, who have been incredibly prolific in making this kind of material available. The book cost 1000 Rs in India and about 4x as much here in the UK, though I've seen copies advertised on Amazon for 4x as much again.
For anyone interested in the script and the history this is a very good book to own. Not only does it have the essay, but it contains examples of Siddhaṃ calligraphy from many eras of history stretching back to the mid 8th century. Many of the familiar images of Siddhaṃ calligraphy have been originally copied from this book I suspect.