How do I get Boethiah's Proving?

There are several ways to find Boethiah's Proving. Although, you must have completed Dragon Rising and be higher than level 20 (For reference)

• At your level, it is possible to be attacked by a follower of Boethiah, who is a random follower. You'll know him when you hear someone say "Ahh! A worthy opponent!" He's an easy kill, then loot his body.

• You can also find it at Septimus Signus' Outpost north of Windhelm

• Its also possible to find it at Hob's Fall cave and the Abandoned House of Markath

The first option, however, is your best bet. A follower of Boethiah is likely to attack you.

The Skyrim Wiki gives us a great deal of information on this sort of thing. To provide context for users that may seek this answer, later, you must be at least level 30 before Boethiah's Proving will spawn, in the game.

As a Drop from Boethiahs Cultist

First and foremost, the book is suppose to drop from a Boethiahs Cultist. While I can not confirm the item has a 100% drop rate, I would speculate that given the nature of the cultist showing up, and the given context, you are simply experiencing a bug. Skyrim is full of them - its a trade off for having such an immersive game.

Found at various locations

• The abandoned house, in Markarth
• Castle Volkihar
• College of Winterhold (appears you have to purchase it from the librarian)
• Hillgrund's Tomb
• Hob's Fall Cave
• Septimus Signus' Outpost
• Apocrypha

Through the command line

If you are playing on computer, you can add any item to your inventory with the right command line. Since you start the quest by reading the book, "cheating" it into your inventory should not disrupt this mechanic. I can not confirm what the correct command line is, so I will have to leave it up to another user to fill this in.

Wait a minute, I don't really need the Boethiahs Proving, afterall!

The main reason you would want Boethiahs Proving (disregarding for cosmetic reasons or for the purpose of lore) is that it initiates the quest Boethiah's Calling. Keep in mind that the book acts as a pointer to the Shrine of Boethiah. If you happen to find the shrine, by yourself, the quest will just start at the next step.