Yuck!
From the BBC:
Thousands of inserts are to be placed into library books to promote the clients of advertising agencies.
The scheme offers advertisers 500,000 inserts in county libraries such as Essex, Dorset, Somerset and also Bromley, in Kent, and Leeds.
Yuuuuuuuuuuuuck.
Then again, there is potential comic value:
"Any product placement would have to be incredibly sensitive - in children's books, for example, or with adverts for something like a funeral director.