I've met a staggering number of people who never knew this existed, so here goes:
Amazon has Top 100 charts for all of their genres, and there are two types: Top 100 Paid, and Top 100 Free. I've been using this ever since I got my first Kindle over five years ago. What I like about just checking this link is that you always end up with free content - whereas with specific deals in this forum, you often get to it too late and the price has already gone up again. Here: it takes you straight to the freebies, every time.
Here are some links to keep bookmarked and check on from time to time, as content changes.
Top 100 Best Sellers in Kindle eBooks [amazon.com]
From there, you can use the menu on the left to sort by genre and sub-genre.
Top 100 Nonfiction [amazon.com]
Top 100 Cookbooks, Food and Wine [amazon.com]
Top 100 Reference eBooks [amazon.com]
I'm not going to list ALL of the categories because it's all a matter of personal preference, and they're all listed right on the page of the very first link in this post. To each his / her own!
edit: it appears that this may not work on the mobile site; so keep these bookmarked on your laptop/desktop.