42
At least according to:
Adams, Douglas. The Hitchhiker's Guide to the Galaxy Book Club ed. Vol. Harmony. New York: 1979. Print.
On a more serious note, here is what a search in all Fintel Library resources discovers.
In the Fintel Library catalog, a subject search for "Life" finds over 100 books and e-books on the topic. Likewise, a search for "Meaning" reveals over forty books on that particular branch of philosophy.
Or, for something completely different.