Over my years of binding and conservation I have come to the conclusion that there is no more important question to ask the person repairing your book than this:
There are always reasons not to do a particular repair and, hopefully, there are better reasons to go through with the proposed work. One always needs to make sure that the conservator or repairer is fully aware of the potential problems down a particular line of repair. Otherwise they might be doing work only because it is what they know to do, rather than being the best solution for the problem.
This could have a detrimental effect on the book, and can end up harming the item being repaired. A repair which might be perfect for one book might be disastrous for another. And it is much more difficult to repair a badly repaired book than a book damaged through use.
Not all books are the same, and not all books are damaged in the same way, so it is very important that all treatment decisions be made specifically for an individual binding.
Knowing the reasons for and against a repair allows both you and the repairer to be satisfied that you both made an informed and correct decision.
Some of the questions to be asked before a repair is begun are:
Why and where did the binding fail?
Why did the original binder choose to bind it the way they did? Did they make the right decision?
Should the structure of book be changed? Will doing that change how the book feels and functions? Does that matter?
Is the item a book or an artifact? See the rebinding page for a discussion of this topic.
Should it be repaired at all, or does it only need a drop spine box?
Will the repair add to the useful life of the book? What is the value of the book, either financially or personally? Will it add or take away from the value of the book? (You can check the financial value at www.bookfinder.com).
How much of the original binding and other elements can be saved and reused?
Should the repair use the original materials (for instance, should leather be used to repair a leather binding or something else)? How will this choice impact the overall binding?