I used this type of sketchbook for urban sketchbook numbers 4 and 5. It is from Jackson’s Art Supplies in Dalston and by mail order online.
It is 9½ inches by 6½ inches and about an inch thick. Inside is proper watercolour paper, a very slightly off-white. It has a texture, but not much: “cold pressed”. Also it is not very absorbent, which is a good thing for fast sketching. This is compared to, say, Saunders Waterford paper which absorbs a lot of colour, I find, and so takes a lot of brush strokes to cover an area.
The cover is a good stiff board, and covered in black linen-like fabric.
I put a paper cover on, so as to give myself a storage flap on the front and back covers.
The biggest available bulldog clips are only just big enough. These have jaws which go to 2.5cm wide and the clip is 5.1cm wide. If anyone knows where I can buy larger bulldog clips, please tell me.