It's possible you're thinking of one of these things:
* Megabytes (MB): A unit of digital storage. Five books would likely take up several megabytes of storage space, depending on the length and format of the books.
* Mega-books: This is not a standard term, so it's hard to say what it might mean. Perhaps you meant "epic" or "large" books?
* A specific book series: There might be a series of books called "Meg" or "Mega" books, but I need more context to know for sure.
Could you please rephrase your question or provide more information?