This only becomes a problem if another bookchain reader is waiting for that book. Don’t hassle the first reader – there may be a very good reason why it is taking them a while to read the book. Better that they finish the book than have it taken from them. However, do make sure that they know they don’t have to battle on with a book they really don’t like.
Try to access another copy of the book for the waiting reader – and give them a spare comments sheet. They will get to see all the other readers’ reactions when the comments are displayed on the noticeboard, and if the first reader returns the book before the second reader has finished, you can always post the comments sheet onto them.
Aim to be flexible with the bookchain rules and always look for solutions that will keep all readers satisfied. If you know there will be a long book in the chain, try and anticipate the problems this may create and reserve another copy at the outset.