Looks like an encoding issue to do with the book title or author. Can you post an example book?
The whole encoding thing was something that come up in the very early days of this plugin. As a quick recap, Amazon.com would do nasty things when it was submitted foreign book titles/authors using UTF-8. It was actually Kovid who suggested using latin-1 for the Amazon website, and until you posted this it seemed to be working well
I am sure Kovid could advise on this but it looks to me like your title contains a character that is not valid in the latin-1 charset. So a simple edit of the title would be one crude way to work around it.
We really need to see some examples. Perhaps there could be some sort of "clean" function applied that strips any non-valid characters for the chosen url encoding out before executing the search. I know Calibre has a function I used for cleaning non-valid UTF-8 characters out from another plugin, but not sure if there is/should be an equivalent for latin-1...