Here is a super duper easy gift idea that you can make inexpensively and in bulk. This is one of those things (if you have jewelry supplies around) that you can put together at the very last minute.
Leather cording, or ribbon
End caps (the kind used for ribbon jewelry)
Using the beads that you choose and simple loops (find instructions for making simple loops HERE) make a few simple loops with beads and charms . Attach the ribbon end cap to the end of the leather cord or ribbon (just fold one half over the cord and then fold the other half over the first), open the jump ring, put simple loops that you made on the jump ring and attach them to the end of the leather cord or ribbon. Close the jump ring and repeat on the opposite end.
NOTE: When making a book thong keep in mind what kinds of books your recipient reads; paper backs call for a shorter cord and hard bound books would require something a bit longer. Find a paper back book, for example, and lay your cording inside and out the top and bottom to give you an idea of how long your thongs should be.