Not necessarily academic books on math (I think you could try out some Coursera courses on math for that as wel).

If you are a developer and want to better understand algorithms both from the theory and practically on how to apply them when, then I can recommend this book: https://www.amazon.com/Algorithm-Design-Manual-Steven-Skiena/dp/1849967202

For general curiousity regarding math these are recommended:

https://www.math.ias.edu/avi/book https://press.princeton.edu/books/paperback/9780691164076/how-to-solve-it https://www.amazon.com/Book-Proof-Richard-Hammack/dp/0989472108/ https://www.amazon.com/Introduction-Mathematical-Reasoning-Numbers-Functions/dp/0521597188 https://www.amazon.com/How-Prove-Structured-Approach-2nd/dp/0521675995/ref=pd_sim_14_3?_encoding=UTF8&psc=1&refRID=M5WBGMT35PP5G3CJT8Q https://www.amazon.com/How-Study-as-Mathematics-Major/dp/0199661316/ref=pd_sim_14_19?_encoding=UTF8&psc=1&refRID=M5WBGMT35PP5G3CJT8QPP https://www.amazon.com/Pea-Sun-Mathematical-Paradox/dp/1568813279

Maybe some among them that you will find interesting :)