Those adhesive covers are really convenient, but I always like non-permanent options better! If your mum can find a reasonably cheap roll of the regular, non-adhesive mylar/polyester, it's sturdy enough and she can use the rest to make more book covers if she has time/more books she want to protect! Or if she's not planning to use it outside her home, maybe a box she can keep it in while she's not using it, like this or this or this?