I have two copies of the book.
its possible that weather conditions,have made it difficult to remove,if it is a problem,here is a suggestion,this must be done with great care.
open the box up,get a hair dryer,and very carefully aim the most gentle heat[not at the reeds]but at the point where the reed pan touches the outside of the concertina,do it in very short blasts,making sure no hot air gets near the reed.
if you are not confident of being able to do this without damaging the reeds,take it to a repairer.
in the meantime,or until you need to sort this problem,if you need to sort it,make sure your concertina is not stored in damp conditions,but somewhere where the temperature is consistently dry[NOT NEAR A RADIATOR EITHER]that will cause even more damage.
two people on Concertina net, Dave Elliot,and Geoffrey Crabb,are experienced repairers,in the meantime dont worry about it,just play the box.