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.
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.
"In considering the use of grammar as a corrective of what are called ‘ungrammatical’ expressions, it must be borne in mind that the rules of grammar have no value except as statements of facts: whatever is in general use in a language is for that very reason grammatically correct."
[Henry Sweet, 1891.]