An interesting book with some practical insights. However, it is serious need of an editor and some stronger theological support.
Like another reviewer said, the text is quite repetitive. A lot of the information in the book is not strictly discussing spiritual warfare, but includes tangents onto other topics (spiritual discipline, for example). This results in a scattered flow to the text, with no clear outline or story arch to the book.
Many of Sherman's claims about good and evil seem to have a basis in his own experience, though he does offer a little bit of scriptural support. However, the scripture used is not extensive, and there is even less historical/theological support for what he writes.
I do, however, appreciate that he was generally reasonable in his views (for example, he was careful not to blame every sickness or natural disaster on spiritual warfare), and I found the anecdotes of his own personal experiences with spiritual warfare interesting. Unfortunately there were only 3 or 4 of these short stories in the whole book.