Source for books on Artillery and Fortifications. Many of these books can be read for free on line = If you search under Google, under the More then Books tags.
http://www.military-naval-history.co.uk/pages/artillery_&_fortifications.htm
Just an example or two:
https://books.google.com/books?id=B...page&q=shot penetration pounder earth&f=false
https://books.google.com/books?id=vylEAAAAYAAJ&printsec=frontcover#v=onepage&q&f=false
http://www.military-naval-history.co.uk/pages/artillery_&_fortifications.htm
Just an example or two:
https://books.google.com/books?id=B...page&q=shot penetration pounder earth&f=false
https://books.google.com/books?id=vylEAAAAYAAJ&printsec=frontcover#v=onepage&q&f=false