Here are my five nominations
1) The Complete Sherlock Holmes (mystery)
Author: Sir Arthur Conan Doyle
Status: Free; public domain
Available at: Mobile Read Forums, various formats
2) 1984 (science fiction)
Author: George Orwell
Status: Free; public domain in life plus 50 countries
Available at: Project Gutenberg Australia
3) The Wizard of Oz Series (YA)
Author: L. Frank Baum
Status: Free; public domain
Available at: Mobile Read, Project Gutnberg and elsewhere in various formats
4) Wuthering Heights (literature)
Author: Emily Bronte
Status: Free; public domain
Available at: Project Gutenberg and elsewhere
5) The Mouse in the Mountain (mystery)
Author: Norbert Davis
Status: Free; public domain
Available at: Mobile Read, Manybooks.net and elsewhere