* Number of words: This would be difficult to determine, as many ancient texts don't have word counts.
* Number of pages: This depends on the size and font of the pages.
* Physical size: This would make a book with larger pages "longer," regardless of content.
That said, here are some contenders for the "longest book" title:
* The Mahabharata: This ancient Indian epic poem is estimated to have over 100,000 verses and is considered one of the longest poems ever written.
* The Guinness Book of World Records: This book is constantly being updated and added to, making it a strong contender for the longest book by page count.
* The Encyclopedia Britannica: While it's now mostly digital, the printed version of the Encyclopedia Britannica was very extensive.
Ultimately, there's no definitive answer to your question. The "longest book" depends on how you define "longest."