There are so many possibilities… I believe that every collector has their own system on how they organize their stamps!
I am really small scale, and far from professional, collector so I “only” have 4 books so far and my stamp organization might be bit unusual… One book is for Nordic countries, one is for other countries, one is “anything interesting” book where I keep very old stamps, stamps from countries that doesn’t exists anymore, some curiosities I have come across, some smaller theme groups and specific series I collect (stamps with women, EU and so on), mini-sheets and so… That book is really little of everything! And then there is the forth book where I keep horse themed stamps.
Nordic stamps are placed by country in more and less random order: Finland, Sweden, Norway, Denmark, Iceland, Greenland and Faroe Islands.
Other countries book is also including countries in even more random order, although I try to keep the continents and neighboring countries together.
With these two I am going with “make it look nice” method, so first the stamps are organized by country, but then in the country section the stamps are organized in way that they look… well, nice. They are loosely in theme groups (anniversaries, nature, famous people etc.) and I keep series together, but then I am also trying to make the each page look interesting and appealing on eye. Sometimes it works, sometimes not so well.
Horse stamps book is separated into sections: portrait style (horses without humans or equipment), horses in art, horses in history, horses in war, carriage horses, horses with riders, horses of sport (all types), and horses in objects (different horse themed items, horse equipment without the actual horse and so on). On each section I am once again trying to make the page looks nice and interesting. I like to keep series together so if the same series have different type of horse stamps, I will place them all into category they best fit in.
With the forth, “anything interesting” book, the organization is more theme based and countries and styles are mixed. There are also different series I am trying to complete. I still try to make the pages look nice, but that book is still in progress and I haven’t found time to focus on it yet.