A book about the constant change in language.

[jim90]

Hello to you all,
 
I'm just approaching my second year of an English Language course; and have become aware that there are no unchanging absolutes in language. I would like to read something 'authoritative' about this matter and would appreciate any advice or information about what I should/could be reading.  

Regards
Jim90

[Alan Palmer]

I'd suggest starting with something by David Crystal. He is acknowledged as an expert on the evolution of English, and his many books are readable by a layperson. His Wikipedia page has links to various sources such as his home page and blog.