This book has long been out of print. Oxford published only 1000 copies. There remains interest in its material, as issues of visibility remain central to many areas, particularly sensor networks, wireless networks, security and surveillance, and architectural design. When I noticed that used-book sellers were offering it on Amazon for as much as $1847 (and at one point, EUR 2151,90 at abebooks.com!), I sought to obtain the copyright from Oxford, which they graciously returned to me. (Thanks to Stefan Langerman for suggesting this, and to Erik Demaine for suggesting scanning it.) So now I can offer the book free to anyone on the web. (But please note I retain the copyright, and all copyright laws apply.) I had it scanned by Kirtas Technologies (they did a great job!*), and the book is linked below in PDF: both the full book, and each chapter individually in case you want just a portion of it. The PDF files are searchable in any PDF viewer that supports text searching.