Two more things.
First, books (including professionally published ones) vary greatly in terms of length. Harry Potter for example goes between 70k and 180k I believe, depending on the book. LoTR is roughly 140k each in comparison.
Intended audience changes that a lot, though remember that eastern light novels are only half to a third the length of typical western paperbacks.
Second, I'm of the opinion that books refer to the physical thing (or e-books lately) and are published (self or professionally), while volume is more fluid and can refer to any book-like delineation and division.