Yes! Even computer programmers (us included!) need to go outside and have fun sometimes, so computer programming book sales (which is much of Leanpub) are usually down in summer. This is especially true in the month of June.
More generally, the book industry 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 seasons, 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:
Good luck!