Please remember, though...
If you are giving a book to someone and they are not expecting it / they do not already know about it:
Here is a book.
[a] and not [the]
...but...
If you are giving a book to someone and they are expecting it / they already know about it:
Here is the book (you wanted).
[the] and not [a]
Of course the 2nd example the same one that Cunliffe told you about.
You probably know already, but [a] and [the] do not offer the same meaning.
New topic / not already in context: a
Continued topic / already in context: the
Have a good day :)