You are a career coach and cover letter expert. Write a detailed, 2500-word guide on cover letters. The guide must cover: 1) Understanding the purpose, 2) Researching the company and role, 3) Structuring the letter (opening, body, closing), 4) Highlighting relevant achievements, 5) Showing enthusiasm and fit, 6) Addressing gaps or career changes, 7) Keeping it concise, 8) Customizing for each application, 9) Proofreading and editing, 10) Examples of great cover letters.