books on functions I am looking for semi technical books on functions, especially ones that can describe natural phenomena (eg Hill function). Maybe not like a reference book but with some descriptions and graphs as well. 
 A: Maybe, you would like to check some books on mathematical modeling. I would suggest some more practical books
You can check "Mathematical Modeling: Applications with GeoGebra" here: https://www.amazon.com/Mathematical-Modeling-Applications-Jonas-Hall-ebook/dp/B01H2KNFK2
"Introduction to Mathematical Modeling and Computer Simulations" here: 
https://www.amazon.com/Introduction-Mathematical-Modeling-Computer-Simulations/dp/1138197653
You could search for partial differential equations books, but this field is quite demanding on mathematical background. If you decide to do so: "Partial Differential Equations I", you can find it here: https://www.springer.com/gp/book/9781441970541   you will find things such as waves and heat equations.
You can also search for some books on mathematical physics or mathematical engineering. (also very demanding)
Finally, you can also check https://www.youtube.com/channel/UCYO_jab_esuFRV4b17AJtAw it has some explanatory videos on mathematics that you might find useful.
I hope I was helpful! Good luck with your studies!
