A brand-new ecash section just went live in the Bitcoin Design Guide, courtesy of nostr:npub1zqsu3ys4fragn2a5e3lgv69r4rwwhts2fserll402uzr3qeddxfsffcqrs. It covers all the basics, and dives into Casu & Fedimint, as well as best practices for design. Take a look. https://bitcoin.design/guide/how-it-works/ecash/introduction/