If you actually want to learn music theory for the purposes of playing I strongly recommend you just follow whatever tutorials for your instrument and style and if you get to the point where feel you want to learn more and take it seriously in and of itself, buy “The Jazz Theory Book” by Mark Levine[1]. In spite of what it says, there really isn’t any such thing as Jazz theory - it’s just music theory, and this book is a great presentation with lots of good examples. If you like jazz (or the western popular music tradition including rock, pop, funk, etc) you’ll come out understanding how that music works better than most people. If you’re doing a music degree, there will be books recommended by your music programme, but honestly, Levine will cover most of what you need if you’re not doing a straight classical course in which case you’ll want to augment with a more conventional source so you understand the “rules” they want you to follow when writing pastiche counterpoint etc.
If you want to get big into composition and/or want to understand late romantic and 20th century music, get Harmonielehre and “Structural Functions of Harmony” by Schoenberg. Together they are by far the best harmony books I have ever seen and go super-deep into why the western classical tradition works the way it does. If you compose there’s a decent chance they will change your life forever as his approach is to not take anything for granted but exhibit everything with examples from masterworks of the western tradition. Amazing books both of them. Harmony is not a spectator sport though - to get full benefit you have to get the books and work through stuff on the piano. It will reward you immensely.
If you want something else that will blow your mind, read “The technique of my musical language” by Messiaen. If you read Levine first this will have the additional benefit of showing you where the “diminished” and “augmented” scales and the whole “harmonic major” thing come from.