I'm having a hard time finding an answer to this question, and was hoping I could find some help here. So, what's the best music theory book for a beginner looking to progress to an advanced level.

Online resources are also welcome; really, I'm looking for anything that's helpful for a student.

Thank you!

I like Mark Sarnecki's materials:

http://www.sheetmusicplus.com/composers/Mark-Sarnecki/14848

Depending on what you're looking for, the Complete Rudiments or Harmony books might be best.

I would recommend Tonal Harmony by Stefan Kostka [http://www.amazon.com/Tonal-Harmony-Stefan-Kostka/dp/0073401358/ref=sr_1_1?ie=UTF8&qid=1360609458&sr=8-1&keywords=tonal+harmony] this is the sixth edition but the seventh is out i just cant find it. Or Harmony and Voice leading by Edward Aldwel [http://www.amazon.com/s/ref=nb_sb_noss_1?url=search-alias%3Daps&field-keywords=harmony+and+voice+leadingl]. I have used both to understand music theory. It used for up to 3-4 different music theory classes. Both really explain the subject so much better than anything else I have read. Good luck its a lot of fun.

musictheory dot net is probably the best free one I have come across to lay a foundation. I would master that first then go to a textbook. Music theory textbooks may be a bit dry for the uninitiated.

I teach with the Sarnecki materials -- the ratio of examples to practice exercises is right for me, and I like how he explains the concepts.

Walter Piston's book is still good, after all these years. Don't know the title offhand though, but you can easily find it.

Tomasino

Although I already knew most music theory, I have recently purchased Robert Greenberg's Understanding the Fundamentals of Music because I find his courses great, and so far I am happy with it. It is not a music theory course in the traditional sense anyway.

http://www.thegreatcourses.com.au/tgc/courses/course_detail.aspx?cid=7261

Mark Sarnecki is really good, he goes in-depth (but sometimes makes things more complicated than they really are):
http://www.sheetmusicplus.com/composers/Mark-Sarnecki/14848

Grace Vandendool is also good; she structures things differently:
http://www.sheetmusicplus.com/composers/Grace-Vandendool/15686

What are the "best" tires to have on your car? What is the "best" part of the world in which to live? It really depends upon your expectations and desires.

Tomasino has already mentioned the Walter Piston text HARMONY, and if you are looking for a comprehensive resource that covers it all, that is the one. The single shortcoming with the Piston is that he does not treat much in the neo-classical, twentieth-century realm. However, to truly understand neo-romantic, neo-classical, and serial techniques, one will need everything in the Piston text as a foundation on which to build.
Ed

Anyone knows a good, pedagogically good modal harmony book?

Dolmetsch is not bad for being free.

If you want "Pop" music theory, Piston may be overboard.

Try Mark Harrison's "Contemporary Music Theory"

I liked these books too. They are not big books, but if you master them both you have a solid foundation.

I thought Walter Piston had the market cornered. I asked the same question over on CMG and somebody came up with Xenakis. The Piston Counterpoinr book is also good - we used it for advanced in graduate school. Roger Sessions has one out there (I used to live in his house...osmosis, you know)...

I bought a second hand copy of my college music theory textbook (Allan Forte, Tonal Harmony in Concept and Practice) for next to nothing.

The individual who originally bought the book appears to have never used it.

I specifically like the Forte textbook for the keyboard exercises at the end of most chapters. Those exercises alone are worth what you might pay for this textbook.

As far as free internet resources are concerned (you'd be surprised how comprehensive some resources prove to be), I've worked (and still do) - and encouraged other members, as well - to compile the best in this thread here: http://www.pianoworld.com/forum/ubbthreads.php/topics/1948785.html

I think he still does, much like Harold Gleason has the organ methods cornered. Stainer was used for many years in the UK, but Gleason's method is... well there is simply no comparison.