I like this thread. I usually have one non-fiction one fiction going depending on my mood and energy levels. As a history major turned back to school engineering major I have a pretty diverse interest in a lot of non-fiction.
But recently I decided I should learn a bit about biology, as that's really the only main branch of science I don't have rudimentary knowledge of. Rather than learning about cells and stuff that I don't think I'd find too interesting, I decided to focus on evolutionary biology, which involves both history and geological scale timelines of which I'm studying (mining engineering major).
A Brief Guide to: Charles Darwin - His Life and Times
I started with a quick survey of Darwin's life, 300 pages, was on sale for $6 at Chapters and an easy read. I was surprised at how well the "Brief Guide" books are researched and cited, as I found a pretty solid bibliography at the back which allowed me to confidently read this without too much trepidation.
Now I'm slowly sifting around his Origin of Species when I feel like it (can find it for free on your Kobo or lots of places on the internet). For a book written in the 19th century it's pretty easy to follow his writing.
The Selfish Gene - Richard Dawkins
Next I've started on Dawkin's The Selfish Gene, written in the 70's, it was the book that put Dawkins on the map. Good read so far albeit I'm not too far into it. The 30'th anniversary edition has a forward in which he says most of the book still stands up today, which made me confident in reading it without the danger of learning something that has since been completely discounted.
A People's History of the United States - Howard Zinn
This was a great read especially as a Canadian History major who studied little to no American history. I wanted a book that gave me a good survey of the big American events in history, rather than getting a book each on the independence, civil war, etc. This book was especially important when it was released because it gave insight into how these great events impacted the common people, instead of just focusing on the great men of history. John Keegan's the Face of Battle was also a great example of this and one I'd highly recommend if you're into military history.