The Tirunelveli District Central Library in Tamil Nadu has the original book in sanskrit with Englsih translation by Sri P.K. Sundaram published by the Centenarian Trust in 1997. The Trust itself does not seem to have a copy of the book (neither the digital nor the printed one).I got a photocopy of the book from that library through the very sincere efforts of one Sri Muthukrishnan, who was an Assistant Librarian there. The cost of the photo copy of the book (including spiral binding in 2013 was Rs 155. You could locate this library though google search if you are serious about the book.
Mr.Jayakumar, Librarian, Tirunelveli District Central Library,
2/32 North Highround Road, Palayamkottai, Tirunelveli, Tamil Nadu-627002
India Telephone: 0462-2561712
You could post your query for the book with the following details and address it to Sri Jayakumar, Libraian at the following website link
The CALL NUMBER OF THE Siva Gita BOOK by P.K.Sundaram in this library is: Q23:25 N7 and it is in the LLA category of collections. SUNDARAM P K
The Library's record says that it was published by Giri Trading Agency (it has branches in many parts of India, but the information is wrong. It was published by the Centenarian Trust)
SIVA - GITA 97 - GIRI TRADING AGENCY 1997