Reddit mentions of Concise Introduction to Tonal Harmony

College does teach music theory but theres a lot of resources elsewhere.

the basics

https://www.musictheory.net/

more advanced

http://openmusictheory.com/

for more serious people

https://www.amazon.com/Concise-Introduction-Harmony-Poundie-Burstein/dp/0393264769/ref=sr_1_2?ie=UTF8&qid=1504284834&sr=8-2&keywords=concise+introduction+to+tonal+harmony

And an important part of learning music is ear training.

Its recommended you have a piano or guitar, or even your voice, and do ear training exercises

https://www.musictheory.net/exercises/ear-interval

I found this guy in particular to be the best source for a video series on the basics.

https://www.youtube.com/watch?v=ICDPWP6HUbk


College degrees on the other hand is way more than music theory. the actual music theory credit classes will usually account for less than 10% of the entire degree.

https://www.amazon.com/Concise-Introduction-Harmony-Poundie-Burstein/dp/0393264769/ref=sr_1_1?ie=UTF8&qid=1550170404&sr=8-1&keywords=concise+introduction+to+tonal+harmony

https://www.amazon.com/Concise-Introduction-Harmony-Poundie-Burstein/dp/0393264769/ref=sr_1_2?ie=UTF8&qid=1517892245&sr=8-2&keywords=concise+introduction+to+tonal+harmony