archive.org has a wealth of out-of-copyright material. One such title is Rolland Allen’s classic text, Missionary Methods: St Paul’s or Ours (London: Robert Scott, 1912).
You can read it online or download it to various formats including pdf and mobi (for Kindle).
We’ve added this to our ever-expanding Bible and Mission books and articles page in the resources section. Check it out for loads of resources: Bible and Mission books and articles