You're welcome
Thinking about the yes/no column I think what I might do is if you create from a boolean column called for example 'To Read', then I will create a collection called 'To Read' on the Kindle and put in it every book marked True. I'll ignore any book marked False/none. Much better than True/False. Although I could create a 'Not To Read' or 'To Read - False' collection and put every book that is false into it. Hmmm....