Aaron Hernandez -- Secret Jailhouse Messages ... FOR SALE

Do you like mystery novels? Do you like mystery novels that have secret jail messages from Aaron Hernandez to other inmates hidden inside them? Then you might be in luck ...

We broke the story ... Hernandez was investigated for trying to sneak notes to other inmates at Bristol County Jail in 2013 by allegedly penning messages inside jail library books ... which the other inmates would then check out. It's pretty Shawshankian stuff.

Now, one of the books that Aaron allegedly used to pass the notes has hit eBay -- asking price? $2,000. The seller is keeping the contents of the note secret as part of the sales appeal.

The seller, who wishes to remain anonymous, tells TMZ Sports he got the book from a guy who managed to sneak it out of the clink. More Shawshankian stuff.

And get this ... the book is "Bad Boy Brawly Brown" by Walter Mosley ... a crime novel about the fictional Easy Rawlins ... a man who faced murder charges in a previous Mosley story.

In the books (spoiler alert), Rawlins was able to prove his innocence and beat the rap.

Wonder if Hernandez chose this particular book hoping life imitates art ... or if it just happened to be the only writing he could grab.