Levine is a great resource, but music theory, and jazz theory in particular, is a super dense subject. Having an experienced teacher to guide you through and help you internalize it is very valuable, even if it's only for the occasional lesson.
Definitely agree about the teacher bit, if you can find a decent one, or can afford it. I find that sitting in or jamming, or even playing over standards is really helpful just to get things going. You have to remember that a lot of the jazz greats of the 20th century had little to no formal music education, let alone the complexities of jazz theory.
