Break down the letter into short, but specific, paragraphs. Don’t ramble, but do flesh out your key points. Try to keep it at one page, covering the various aspects of their recommendation in brief, specific detail.
Do graduate schools care if letters are confidential or not? In general, graduate programs prefer confidential letters. Admissions officials say that it displays more confidence on the part of the applicant if letters are "confidential" (meaning you, the applicant cannot see the letter). The Letter Service is required to indicate on the letter if it is confidential or non-confidential.