I was browsing and found a few HarperCollins science and natural history books on sale for $1.99. I've linked eBooks.com and Amazon, but I expect these to be on sale pretty much everywhere.
A Cultural History of the Elements, from Arsenic to Zinc by Hugh Aldersey-Williams
eBooks.com Amazon
The Oysters of Locmariaquer by Eleanor Clark
eBooks.com Amazon
What Should We Be Worried About? Real Scenarios That Keep Scientists Up at Night by John Brockman
eBooks.com Amazon
Why Science Does Not Disprove God by Amir Aczel
eBooks.com Amazon
A Garden of Marvels: How We Discovered that Flowers Have Sex, Leaves Eat Air, and Other Secrets of Plants by Ruth Kassinger
eBooks.com Amazon