The recommendation below is an advertisement. Most of the links here are part of Amazon's affiliate program (unless otherwise stated), which helps support this website. It's also more than that - it's a hand-picked recommendation of something I've used myself. If you're interested, here is a full list.
Cottonwood Arts Sketchbooks
These are my favourite sketchbooks, hands down. Move aside Moleskine, you overpriced gimmick. These sketchbooks are made by entertainment industry professionals down in Los Angeles, with concept artists in mind. They have a wide variety of sketchbooks, such as toned sketchbooks that let you work both towards light and towards dark values, as well as books where every second sheet is a semitransparent vellum.