On the off chance that the authors read this: please, since you have no current plans to finish the project, make the document source code available (I'd guess it's LaTeX, but there's more effort put into layout than usual) and release it under an open license. For something like this (where the document would be very difficult to reproduce without the source file), the GNU Free Documentation License[0] is probably appropriate since it will force anyone redistributing the book to make the source available too.
You might even find people willing to maintain/add to the project (I'd offer to help, but am in no way qualified to maintain a book on PDE).
You might even find people willing to maintain/add to the project (I'd offer to help, but am in no way qualified to maintain a book on PDE).
[0]: www.gnu.org/copyleft/fdl.html