APA (7th ed.) Citation

Kipling, R., Heeley, S., Morris, W., & Burne-Jones, E. C. Autograph letter signed: My dear Sibyl.

Chicago Style (17th ed.) Citation

Kipling, Rudyard, Sibyl Heeley, William Morris, and Edward Coley Burne-Jones. Autograph Letter Signed: My Dear Sibyl.

MLA (9th ed.) Citation

Kipling, Rudyard, et al. Autograph Letter Signed: My Dear Sibyl.