Something new that I've been working on is watercolour journals. Bound in rich, dark brown leather which is simply gorgeous (and my most popular). These books are made with 140lb cold pressed watercolour paper. Very thick and very nice for field painting and watercolour sketches.
I also made these books a bit bigger (9" x 6"), because when sketching, bigger is always better, but they're not too big, so it's still quite easy to bring along with you.