Heat sensitive eraseable pen and a clothes iron
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.)
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.
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.
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.