Here are a few resources that you may want to consider. I am careful with that word and if I've demonized one of God's own, it is only because I very wrongly understood Watts' teachings. If they are what they seem to be, he preaches another Gospel.
I have read these articles. While I agree with most of the assessment, I disagree with the way the author draws conclusions which Watts himself did not draw. At any rate, he makes a distinction between the kind of speculations which Watts made and the dogmatic errors of people who were classified as arch-heretics. They also leave scope for concluding that Watts was quite out of his depth.