Language change: progress or decay
Aitchison looks at how social pressures can change the way we speak, either trying to appear more cultured, or in adopting more traditional ways of speech. She shows how language often changes in a predictable way, with simplification of words that are hard to say, followed by a process of regularization, trying to make a language seem more consistent. She considers how the speech of children, and of those with language disorders, differs from normal speech, but argues that these differences don't play much of a role in more general language change. There are also chapters on how words can change their meaning, and on the birth and death of languages.
The conclusion is that language change is not the process of decay that many fear. However, I felt that there are some things which the book doesn't really address. For instance, how much does the huge amount of material in written form slow down the process of language change. Aitchison also indicates that she thinks the politicization of language to be more of a problem than language decay, but doesn't go into detail. So the book might not answer all of your questions, but it does provide and interesting and informative introduction to language change, written in a way that is accessible to those with no previous experience in studying linguistics.