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Maybe the ability to create your own online affiliate book shop, that might be dedicated to a certain topic, or even try to push a certain set of books.
You also want a special review block, that allows XOOPS Admin to combine existing online reviews, with reviews from XOOPS site users, combined with a rating system.
And for the future you might want to keep in mind the option, allowing admins to change the used web service (e.g. switch from amazon backend to b&n).
Further, offer the admin the option, to assign users rights to books individually (as you are offered in
XOOPS Gallery).
Just my ideas.
Great project! All the best for its succeeding!