* By word count: The current record for the longest book by word count is held by "The Book of Records" by Robert M. Price, which contains over 13.5 million words. However, this doesn't necessarily translate to the most pages.
* By page count: The longest book by page count is hard to determine definitively as there are many books with thousands of pages, and the size of pages can vary greatly.
* By physical size: The largest book in the world by physical size is "The Book of Kells," a beautifully illuminated manuscript that is 330 mm by 250 mm, but it's not the longest in terms of words or pages.
Therefore, it's impossible to definitively say which book is the "longest" without clarifying what you mean by "longest."