As far as I know, Mal’s letter will only be available in the paperback.
HOWEVER…
1. Don’t tell my publisher I said this, but you could just pop into a bookstore and read the bonus content there. (And then perhaps buy an extra copy for your dearest friend or a penpal in Boise or that nice fellow at the coffee shop.)
2. Right now, there are two separate giveaways running for the Shadow and Bone paperback on
PubCrawlBlog (where you can win additional exclusive bonus content)
and
YA Bibliophile (where you can also win Shadow and Bone swag)
I hope that helps a bit?