Suggestions on improving calibre in any way, including usability are always welcome, as long as you remember that your suggestions may or may not be accepted, depending on the judgement of the calibre developers involved, which usually means me
Some things to keep in mind:
1) calibre's interface is the end result of almost a decade of steady improvements based on constant user feedback. That is not to say that it cannot be improved further, but, if the interface does something in a particular way, there is almost always a reason for it.
2) The specific trumps the general. This means that
a) Specific suggestions are much likelier to be useful rather than general ones. So for example: "Change the wording in dialog X to be Y is useful". "The wording in the dialogs should be made clearer" is not.
b) Many UI "experts" tend to have a bunch of general principles that they advocate with out taking into consideration how those principles apply to the specific situation, or whether there are special circumstances that trump those considerations. I'm not saying *you* will do that, but it is a tendency to watch out for.
3) Any given suggestion has to be implemented by a real human being, who is likely to have many other demands on his time. Keep that in mind when making suggestions.