PLAI's license is CC BY-NC-SA (3.0 US). I have not yet decided for sure with PAPL.
At any rate, I'm committed to having a full copy of the book available for free to all, as is also true of HtDP (we were one of the pioneers in this space) and PLAI (which has even more free/commercial options than HtDP).
You may assume at least a CC BY-NC-ND, which lets typical users have at it right away.
With PLAI 1/e, I did something much simpler. I made a pay version of the PDF available, making clear it was also available for free, so people would only be buying it to reward the author. I'm pleased to say some number of people have done so with regularity. Search for "Electronic Version" on http://cs.brown.edu/~sk/Publications/Books/ProgLangs/2007-04....
Could you please change the body text color to something closer to #000, if not #000 itself? The current contrast ratio fails WAG AA and AAA standards for normal sized text.
At any rate, I'm committed to having a full copy of the book available for free to all, as is also true of HtDP (we were one of the pioneers in this space) and PLAI (which has even more free/commercial options than HtDP).
You may assume at least a CC BY-NC-ND, which lets typical users have at it right away.