A biography of Leon Jastremski was written by Edward Pinkowski in 1974. The title is "Pills, Pens and Politics" and was published in Delaware. Sorry, don't have the ISBN.
There is a brief outline of his service and a picture in "Lee's Foreign Legion - A History of the 10th Louisiana Infantry" by Thomas Walter Brooks & Michael Dan Jones. I do own a copy of this book, so if you are unable to find it, let me know and I can send you the information.
Quite an interesting man!