Rite in the Rain All Weather Pocket Notebook (146)
This is a nice convenient size for a hip pocket at 10 cm (4 inches) wide by 14.5 cm (6 inches high). As well as the note sheets, the back also has measuring scales printed on to help you with field measurements of small items.
If you want a smaller version for your shirt pocket then take a look here.
You can also get a Cordura cover for this book.
More information on Rite In The Rain can be found here. And don't forget that these products were just perfectly made to go together with Space Pens!