Is it possible to offer a volume discount for a book?

Yes!

One option would be to make a coupon link for price X, with Y number of uses.

You can make coupons here:

leanpub.com/YOUR_BOOK/coupons

Here's an article on how to create coupons that might be handy:

http://help.leanpub.com/en/articles/640826-can-i-create-coupons-for-my-books-to-give-them-away-for-free-or-at-a-discount-as-part-of-a-promotion

You can also make a package and set the price and number of copies here:

leanpub.com/YOUR_BOOK/packages

That way, everybody sees the bulk purchase as an option on your book landing page.

Here is an article about this:

http://help.leanpub.com/en/articles/3287107-can-i-make-a-package-to-sell-multiple-copies-of-a-book