I recommend Serious Cryptography by Jean-Philippe Aumasson. After reading it, you will gain enough understanding to compose cryptographic primitives and build your own secure system based on well-known best practices, as long as you don't deviate too much from the golden paths. Although with that, you still won't know how to design or implement these primitives yourself. It's like having a nice toolkit of screwdrivers, hammers, spanners etc to build your thing, but you can't build those tools themselves.