For the “Bicycle Boy” illustration series I was using the same sketchbooks as Hayao Miyazaki.
I got interested in those sketchbooks since I saw the documentary series about “Ponyo” in which Hayao Miyazaki was using them to do concept arts with pencils and watercolours. He was also often carrying them around while walking between his atelier and the studio. Naturally I got interested and went to investigate.
They are really cheap (F6 size for about 500¥), have nice quality paper (I used watercolours, pencils, brush pens and even my fountain pen with no problems) and come in many sizes. Each has 20 pages of 156.5g/m2 paper.