A writer's notebook is a case of digging your well before you are thirsty. Writer's notebooks are personal, make of it what you may types of records. Some keep them as journals and others have a specific book just for writer's notes.
Writer's notes are anything a writer wants to keep record of for future reference. These notes are used for sparks, inspiration or a nudge when a writer needs an idea. Sometimes ideas are all around popping up all the time and then sometimes they all run down the idea hole, nowhere to be found. Your writer's notebook comes in handy when the ideas are hibernating. And anything interesting to you qualifies for your notebook. It's yours after all, so don't worry about writing down anything wrong.
So many things are good to write in it. You may not be noticing all the different things you could be. Here's a list of things to be on the lookout for to include in your book:
+ Interesting words you notice
+ Quotes and one-liners from anywhere including people you are around
+ Eavesdropped conversations and ideas for novels from them
+ Headlines and story ideas from newspapers/magazines
+ Character profiles from real people
+ Setting details from real places
+ Things you hear children say - they see the world in different ways
+ Ideas you get while reading
+ Names of places on signs and business cards
+ People ahead of you in line and why they are buying what's in their cart
+ Habits of people around you - do you know why they have these habits?
+ Things you find really pretty or really ugly
Write down things you want to keep, but don't have a need for now. They someday may fit in a story or make a story for you. You just need to be able to reach out and grab them at the right time.