naphtha based lighter fluid seems to remove the adhesive from tapes and pricing stickers from most hard surfaces without damage, but i don’t know how well it would work on the book covers.
I’ve read that freezing some adhesives will eliminate their stickyness. If nothing else, you won’t have to buy anything or risk damaging the book by trying it.
Lots of suggestions here already, but I thought I’d add mine too.
Check out a scrapbooking store (or online scrapbooking supplier if there’s not one in your area). There’s a relatively new product out specifically for removing adhesives. It’s shaped like a pen, but instead of a nub there’s a chisel-type scraper, and the barrel is filled with a dissolving solvent. You just need to squeeze the pen as you scrape gently with the chisel and the sticker/tape/etc. comes up easy as pie.
There are such things as eucalyptus lollies (sweets to you Americans), that taste just like eucalyptus smells (if that makes any sense). Don’t you get these in the US?