For textbooks, you would need to be more specific on what topics you are interested in.
As for books you could find in Barnes & Noble (or similar stores), try The Math Book. It has short blurbs on a lot of really important mathematical ideas in chronological order. Also, the series "A Very Short Introduction to..." is really good.
EDIT: Also, this
Not really, there are maths texts which are a bit like the 'popular science' books you can get, e.g. https://www.amazon.co.uk/Pursuit-Equations-That-Changed-World-ebook/dp/B007P05A1W I don't think they're very interesting but I guess some people must like them.
What are the top mathematicians spending their time on? Well, if we're talking about academic mathematicians, it's mostly very abstract and hard to say. Oftentimes other mathematicians not in the appropriate field of study would struggle to understand the statements of the problems.
That being said, graph theory and combinatorics in general have some understandable problems that you could appreciate, here is one that I was naive enough as to try and tackle! https://en.wikipedia.org/wiki/Erd%C5%91s%E2%80%93Faber%E2%80%93Lov%C3%A1sz_conjecture
I was never a top mathematician, so I can't exactly say, but my supervisor was and is, and he spent a lot of his time reading mathematical papers, digesting the key ideas and seeing if the ideas could be used for other (graph theory) problems. I think this is fairly typical.
I don't think you can really learn about maths without actually just learning maths, the problem is that it is extremely abstract and hard to translate into some vague statement like you can do with other sciences.