I usually suggest Eric Taylor's 2 volumes of The AB Guide to Music Theory because they are small and cheap. And the Associated Board (of the Royal Schools of Music) is highly regarded in the UK.

These books start at the basics, but take you a long way, and some find it daunting. I suppose they take you to grade 8 theory exams, but I never had to go beyond grade 5. And also I guess they greatly condense what would be dealt with in a much more leisurely manner in a classroom.

You could try just volume 1 and see how you get on with it. (or there's also his First Steps in Music Theory, which I've never looked at)

It is sometimes possible to get the two volumes for a US cent each plus shipping, and they are only about 100 pages each, so if they are not for you, you didn't spend a lot of money. However, I've just looked at Amazon.com and they are not so cheap at the moment.

Note, please, that "sight reading" doesn't mean playing from music, it means reading a piece for the very first time only. After that, technically you are no longer sight-reading, you are just playing from music (although if you have a poor memory and sight-read a piece, then play it again a month later, the second time is as good as sight-reading). I don't mean that to sound pedantic, it's just that I've seen forums where there has been great confusion over this.

AndrewH said
Volume I goes through ABRSM Grade 5, and then Volume II goes from there to Grade 8. You can think of Grade 5 as the basic theory that all performers should know; levels above Grade 5 are useful for performers but perhaps of more interest to composers and conductors.

In fact, to do practical exams above about grade 5 or 6, it is compulsory to have passed the grade 5 theory exam (which is why I did it). You don't ever use it - it's just one of those pieces of red tape.

Perhaps they think that if you are going above grade 6 practical, then you may be heading for music college and some basic theory is one of the entry requirements for that?

I'm just musing now - I suppose if you are playing a piece on the piano, it might help if you know what a perfect cadence is, lol!