I recommend the MIT Guide, a very popular source for a majority of individuals wanting to learn about lock picking.
MIT Guide
Sorry its not a book book though, but I believe it is the best source; I'm sure if you search around on these forums there will be newer guides written with fancier pictures and videos, but I still perfer the MIT guide.
Regarding a book, try getting "Easy Pickings" it is a book included with a purchase of a Southord pickset
Easy Pickings
But best bet is to take a stroll on Amazon and search "Lock picking" and read the user comments on those books and see which one fits you.
If you're looking for a free book, I doubt your local library would have any books regarding the topic of your interest.