A Google user
This is the classic mid-19th century work on fortifications by the most distinguised professor of military engineering at West Point Academy, Dennis Hart Mahan. It is key to understanding the system of forts that developed in the early 19th century and provided the backdrop to much of the Civil War. Much of the material in this book was taken from materials taught by Mahan to cadets who later became officers in both the Union and Confederate armies.
Unfortunately, this particular version of Mahan's Permanent Fortifications is nearly useless. Although the text goes into elaborate detail, it is nearly impossible to fully grasp the concepts of the fortifications of the era without reference to the many line drawings provided on the plates that accompany the text. Alas, when this book was scanned, the scanner did not bother to fold out and scan the plates. All that the reader can see is a glimpse of the plates where one leaf was folded over another...enough to tantalize, but not enough that any of the text that refers to the plates makes any sense.