Heat sensitive eraseable pen and a clothes iron
Tools:
Eraseable pen that erases by the heat of friction, (the manufacturer of my pen's claims 65oC.)
Standard clothes iron set on high.
A blank sheet of printer paper. (To protect the iron from any ink as it becomes visible if exposed to cold temperatures.)
Procedure:
Once the iron has heated up, place the paper over a page and go quickly over it as if you are ironing it, about 2-3 pages deep are reliably erased, while sections you covered less begin to appear a few pages down.
Results:
It took about 10 minutes to erase the 50 or so pages I'd filled in a 180 page workbook. It looked like the page may become too warped by the heat, but they seemed acceptable/normal range of my other used books afterwards. On inspection, I missed 1 page fully and some corners of 3 other pages. Other ink, pen and pencil notes were unaffected.
Disadvantages:
Alternate heating methods:
Laminator- really cool and unlikely to set paper on fire without a manufacturer's defect, but only useful for loose leaf paper. (I tested this first to make sure there is a safe temperature for the entire page that reliably erases all the ink.)
Blow dryer- mine seems to max out right around 65oC since it just disappeared in spots, but it looks like a hotter dryer or a heat gun with a paper safe setting would be almost as easy with some finer control.
Oven- I've decided not to try this for for now though my oven does go down to as low as 50oC so it should be possible to do safely.