I used to visit car boot sales regularly. Never saw a book that looked like a menace, but did spot a couple of odd things. One was a clear plastic envelope labelled in English and Greek, "do not open or touch the contents" - it was a dried scorpion. The other was from the 1930s - a radium-boned corset in its original packet:

https://underpinningsmuseum.com/author/lorraine-smith/upmtc-the-curious-history-of-radioactive-corsets-by-lucy-jane-santos/

You have to wonder if anybody ever tried that to make books you could read in the dark.