History of downloaded iBookstore samples?
I was able to recover the list of all books in iBooks, including sample chapters, with the following steps:


Select a tool to parse iPhone/iPad backups on your hard drive. Google for "iphone backup extractor", there are various.
Select a recent iPhone/iPad backup from the list presented by your extractor software.
Select iBooks as the app you wish to extract.
Search for a file called iBooks_v10252011_2152.sqlite, or something like that.
Use a SQLite viewer (such as Base on the Mac) to view the ZBKBOOKINFO table.
Look for the ZBOOKTITLE field - this should contain a list of all your books, including sample chapters, from the time of the backup!
If you want to get fancy, try sorting on the ZSAMPLECONTENT field.