The book definitions of hypomania and mania seem to make the difference between the two pretty unclear. They generally either list hypomania as “mild mania”, or — the other way round — mania as “stronger hypomania”. Then list the same symptoms next to both.
Here in the Netherlands the definition is a bit clearer: mania requires hospitalisation (or arrest, but hopefully the former). Hypomania is what you can cope with by yourself; mania requires help. While this still isn’t a 100% clear division — how are you expected to know, especially when manic, that you have now crossed the threshold and you need hospitalisation? — it’s helpful, because it gives a clear and understandable difference.