Ask five collectors how they organize their stock book and you’ll probably get five different answers, and all five can be right. The trick isn’t finding the one correct system, it’s picking the one that matches how you actually think about your own collection.
Organizing by country works well if you’re a general collector building broad coverage, since it mirrors how most catalogs and supplements are already structured. Organizing by topic, wildlife, space, sports, whatever draws you in, works better if you collect for subject matter more than geography, and it makes a stock book genuinely fun to flip through since everything on a page shares a theme. Organizing strictly by date is less common but useful if you’re chasing a specific era or building toward a particular anniversary or supplement year.
The honest advice: pick one, use it consistently, and don’t feel obligated to switch just because someone else swears by a different method. A stock book you actually use beats a perfect system you never get around to filling in.