If you'd like to do a review for a book that isn't in the review grid, you can make a quick wiki page for the book in the 'books' forum, and then post your review below it, starting with 'i give this book x(number) out of 10 acorns' and then it will turn up in the grid.
The wiki doesn't have to be as detailed or shiny-looking as other wikis, it just helps to get the cover image and copy/paste the blurb up, and maybe a link to Amazon for it.
To make a wiki, just start making a new
thread, and in the tick boxes below the bit you write the post in, there's an option to tick about making it a wiki.
I look forward to reading your reviews.