If you want a real encyclopedia of knots, The Ashly Book of Knots is widely considered the bible of knotting. Clifford Ashley was an enthusiastic sailor and knotter who traveled around the world collecting knots and stories which he includes in this well written and illustrated book. It was by far the most comprehenesive at the time and still excellent today. The disadvantage is that he wrote his book in 1946 and there have been a number of interesting and useful knots discovered since then, which obviously would not be included. I don't know any modern texts that are as good generally speaking, though. I believe the International Guild of Knot Tiers is currently working on making an updated edition, which should be out soon. Hopefully they'll do a good job and we'll finally have one comprehensive modern knot collection.