Here's a couple of slightly different takes.
1. When you know what the book's title is going to be: Go ahead and create the Empty Book BetterRed suggested, then add a tag that says "ToBuy" (or whatever) then you'll be able to use the tag browser to call up all the Empty Books that are ToBuy when you want to review just that list.
2. If you don't know what the title will be, but expect there will be later additions to the series, add a tag to a current book in the series. I use "*FindSequel."
I use an asterisk in front of what I call functional tags so that they'll be grouped in the tag browser. Things like Wishlist, ToBuy, FindSequel, Paper.
And here's an earlier thread (from 2008) called "
Adding Wanted Books"
Hope some of that helps.