I am not a native English speaker and want to improve my letter writing skills. I'm not looking for letters written at an ordinary academic level, but for those that might exemplify the abilities of writers at high professional or educational levels. Therefore I am looking for well written English letters (written for a wide range of purposes) and having a wealth of English (complex, sophisticated, or artful sentence patterns, super verbs, and words). I googled, but found only ordinary letters. Please let me know of any resource.
If you're looking for professional letters, some excellent resources would be to look up letters written for and by Presidents, Prime Ministers, Monarchs and Dignitaries. World leaders are generally among some of the best-educated members of their geographic area.
Elected leaders especially tend to be quite well-written (or employ others to write for them who are); lacking the ability to eloquently communicate with their supporters and persuade their opponents means they are going to lose to someone else who is.