When you request a book or audiobook from a publisher it will be "pending" until it is approved or declined. During this time you can see all your requests that are still pending in the Not Active section of your Shelf.
To find your Pending Requests, click on your Shelf, then your Not Active section. Please note that each column can be sorted by clicking on the column title.
If you would like to withdraw your request on any book, simply click on the Withdraw Request link.
Please note that if a request is withdrawn, you will not be able to request the book again, and you will only see the Wish for it! option going forward. The only way to override this would be to access the book using a widget invite, which you can read more about here.