If you want high-ticket clients, proposals make or break your business. Learn how to write a winner. Most of the time, people don't believe me when I tell them that it's 100% possible to close 80% (or more) of the deals you write proposals for.
Proposal
How to Write Proposals that Win 80% of Deals — The Startup
Jakob Nielsen's 10 general principles for interaction design. They are called "heuristics" because they are broad rules of thumb and not specific usability guidelines.
Usability
10 Usability Heuristics for User Interface Design
Knowledge
How to Maximise Traffic to a Bootstrapped Product Hunt Launch