The book industry in general is seasonal, and our understanding is that this is particularly true for what is known as "prescriptive non-fiction", which is the sub-genre many Leanpub books fall into.
For books in English, the trends follows the northern hemisphere educational season, so things slow down in the summer.
Books in all genres get a boost around Black Friday and Christmas.
To boost sales of your book, we recommend you sign on to Leanpub's newsletter sales, which you can do here:
Author > Books > [Your Book Title] > Store > Pricing
You can also make coupon links to distribute to people online and in your own email newsletter, if you have one:
Author > Books > [Your Book Title] > Store > Coupons
We've got some more advice here: