Some sketchbooks I see on the internet are beautiful, complete books and works of art on their own. In my opinion, this is not what sketchbooks should look like. A sketchbook is a workbook, a place for play and experimentation, and also a place for practice. They should be messy and imperfect. I usually have a couple sketchbooks going at once. One is a 8.5x11 book, and this is where I practice drawing, and also a workbook for ideas - for client and personal projects. I also have a larger 11x14 book that I keep just for life drawing. When I go to Dr. Sketchy sessions, this is the book I use. I find the bigger size is easier to practice anatomy with. Some people also keep smaller travel sketchbooks for sketching on the go. I just find the small size hard to draw in.
Playing and experimenting in your sketchbook is very important. This is where failure comes into play - fail in your sketchbook, not your client projects. If you practice something enough, eventually you will stop failing and start improving, and become good at the thing you are practicing. One thing I have been playing around with lately is brush lettering, and hand lettering. I started doing lettering in my sketchbook and on personal projects such as christmas cards. Now it has started bleeding into my client work - the logo I am currently working on is hand lettered, and also uses brush lettering. If I hadn't played around with lettering in my sketchbook, it would have limited where I could go with my client work.
Keep your sketchbooks playful and use them as a non-judgemental place for drawing and coming up with ideas.