This feature is used to share your books that you've received. They can be bought, from the library, or literally in your mailbox & more. It's a way of sharing books but not in a form of a book review.
Each week or so, i will post books that I've received, which i cannot read them all in a period of time. So i found this really useful, which In My Mailbox can also be "sneak previews" of what book reviews will be coming up soon on my blog!
Books i've bought from Barnes & Noble
(Yes, Literally did come from In My Mailbox LOL):
You Against Me By: Jenny Downham
Boy Toy By: Barry Lyga
Thirteen Reasons Why By: Jay Asher(Currently Reading!)