Field-tested art supplies:
I like the Robert Bateman ones that I buy locally or from Daniel Smith www.danielsmith.com.
They’re made from recycled paper, and the pages are thick enough to hold a watercolor wash. The only thing about these sketchbooks is that they are spiral bound, and if they spend enough time getting beat up in a pack, the cover comes off. I also carry a 3″ X 5″ Moleskine in a ziploc in my work pack. Sometimes I carry pieces of cold-press watercolor paper in the back of the sketchbook, and I’m experimenting with a watercolor sketchbook.