I think it depends a lot on what, precisely, it is you think you need to learn.
There are "standard" texts such as Piston, Fuchs, Ravel, Ottman, Shearer, and others. All great books, and as mentioned, all very expensive. If you are weak on reading I use the Aaron Shearer book for all my students, it is geared towards guitar, but I think it can work for almost anyone.
My recommendation would be to visit
Alexander Publishing. The late Peter Alexander had a teaching style that worked really well for me. You can combine books, including some of the "classics" with audio files, MIDI files, and videos to make the lessons more meaningful.
He covers the entire spectrum, from reading to scales and chords to counterpoint to arranging and orchestration, so it is a kind of one-stop shop for music education. I find the "Spectratone" chart to be very useful, even after several years, although I don't refer to it as often now (which I hope means I'm learning and not lazy!)
Some of his books are used as college texts, and I'm not sure that's a valuable metric, but if doesn't hurt<G>.
Another teacher I've recently discovered is
Dr. Norman Ludwin, he offers several books, and I'm really enjoying his orchestration courses! He has a course on composition that I am going to study soon - well, as soon as I've finished the orchestration courses...
I've been following the adventures of the
Open Music Theory web site - it looks like the real deal, and it does start at the very beginning. I haven't gone through it in detail, but it is definitely worth a look.
I'd also echo DMBaer's comment on finding a teacher - I think anyone with a little drive CAN teach themselves pretty much anything, but I also think that working with a teacher, especially at the beginning of a course of study, is invaluable, and will accelerate the process.
(Disclaimer, I did study music theory in a previous life, so I probably had a bit of a head start. I honestly can't say if or how much that helped, but I suspect it makes a difference.)