WITH A CUP OF TEA AT MY SIDE, SHALL WE BEGIN?

The wait has ended! My book is no longer sitting on my side table, collecting dust...Today, I've decided to start reading How to Read Literature Like a Professor. Before doing so though, it dawned on me that many different art forms may be considered "literature", what with poetry, and lyrics to a song, and short stories.. the list goes on and on. If literature is a written work, would informal letters sent to your grandma be categorized as literature? Must it be a written piece or could dialogue also count? I mean many scripts are seen as "good literature". So, I wanted to define what "literature" really means before I start deconstructing it.
I've found that it is hard to truly solidify a concrete definition for what literature is because it is an abstract concept. Even with just looking at the definition that my laptop provided, it doesn't quite satisfy me. Oxford's definition seems so superficial and bland. Perhaps with reading this book, I will be able to find my own answer to this question. I will make that my mission for this read: What exactly is "literature"?