It is a
Do you want to say something?
Hang on, let me write it for you.
Books record conversations so "let me write it for you".
Remember that in every word you're saying,
I'm playing a part, it's true.
Every word we say we learn from books or someone learnt from some book and we learnt from them.
Want to imagine like, anythin'?
I'll prove it wrong, fool you!
For everything you can imagine, you can find a counter in some or the other book. It might not be true always.
Want to throw me into a bin?
Think twice before you do.
Books give us knowledge which makes them highly valuable.
Oh, wow, now you're a bird
Both of us can count to three
This is like a mock, on the idea of that a book is useless to a bird, so both are useless to each other, hence the mock "both of us can count to three"
I am not really a word,
Yet, can you find me?
It's a book, not only a word.
P.S. : Another answer based on the same reasoning is "knowledge"