Sketchbooks are a great way to store all your ideas and develop your work. They do not need to be a piece of artwork but a place to jot down ideas/notes, a way to save pictures/photos, somewhere to record tests, storage for samples and most importantly place to refer to when you need inspiration.
I use a large amount of my own photographs in my sketchbooks as I have always enjoyed the art of photography in its own right. I collect images of details, textures, interesting forms and anything that grabs my attention. As well as storing these digitally I surround myself with these images alongside my tests and collected images by other artists I admire.