I don’t think there’s a book specifically that tells you “everything” about Mandalorians but there’s a lot of books that sort of give you a lot of the lore. Though if you really wanted to know virtually everything about them, we are on the Wookieepedia, you know.

Abel Pena's History of the Mandalorians and Jango Fett: Open Seasons is a good start.

The Republic Commando series is unfinished, but super good and ties into a lot of Mandalorian lore. I think it is the most underrated series in all of Star Wars.