All missionaries are missiologists (or at least should be). Not all missiologists are missionaries. They often are not.
Missionaries vs. Missiologists
A missionary is a Christian that is sent out of his church to evangelize cross-culturally. A missiologist is a student and often times a teacher of missions. Surprisingly, many of the books on my shelf about missions are not written by missionaries. They are written by missiologists only. They are written by armchair missionaries.
Two of the greatest missionary thinkers of the 19th century were Henry Venn (a British Anglican) and Rufus Anderson (an American Congregationalist). Neither were missionaries.
Dentists (or, former dentists) make the best dentistry professors. Pastors (or former pastors) make the best teachers on the pastorate. And missionaries (or former missionaries) make the best missiologists and teachers on missions. This is why missionaries often struggle listening to missiologists that have never served overseas.
“Missionaries and missiologists, though laboring with the best of intentions, sometimes find each other completely incomprehensible” – Robert Dann, Father of Faith Missions, 468
Non-missionaries can still speak intelligently about missions. John Piper was never a missionary, but his book Let the Nations Be Glad inspired many to cross the globe with the gospel. Roland Allen was only in China for a few years, but his work Missionary Methods is considered a classic in its genre. But there is a kind of depth that only a veteran missionary can give when writing about cross-cultural evangelism.
A missionary author has a special kind of authority, clout, and ethos when he has actually learned a foreign language, moved to a foreign land, lived among the people he is trying to reach, and won converts in a spiritually dark place.
All missionaries should be students of missions but not all students of missions are missionaries. The best books on missions are either written by missionaries themselves or biographies about actual missionaries.