I occasionally have the same issue, especially when the book hits a snag and goes on a topic that is not particularly engaging to me. A low cost way to still make good use of reading time for me at least, is to switch to another book to read. If that book isn't quite doing it for you at the moment, then switch to a tertiary book and so forth. This is particularly easy with an electronic reader.
A this current point in time, I am in the midst of alternating through a rotation of 5 books. Surprisingly, the switching cost of reading in such a way doesn't seem very great to me. When I open a book I haven't read in 10 days let's say, once I read a page, I can recall past content in that book fairly quickly.
