I have two things for you.

The Notebook; A History of Thinking on Paper, by Roland Allen, is an accessible book about the subject in question and might give suggestions for other ways to use your notebooks if you'd like. I'm a notebook keeper (I have a couple, and sketchbooks, I've been working on being better at drawing these past couple of years myself).
I am very fond of the MUJI USA Gel Ink Cap Type Ballpoint Pen .38mm. It's very precise, so it works very well in the smaller Field Notes notebooks. (I do also use a Waterman fountain pen that was a gift back in the 90s on a daily basis, too, but I have no idea what model, and I use it in a Huhuhiro journal that is like a Moleskine. It has been in daily use for more than half of my life! What a champ of a pen.)