Complex sentence: The Mahatma Letters are letters that were claimed to be written by the Theosophical Mahatmas or Masters of the Ancient Wisdom to certain Theosophists, especially A. P. Sinnett, A. O. Hume, Henry Steel Olcott, Helena Blavatsky, C. W. Leadbeater and others.
Split into simpler sentences: The Mahatma Letters are letters that were written by the Theosophical Mahatmas to certain Theosophists, especially A. P. Sinnett and A. O. Hume. Letters by the Mahatmas were also written to Henry Steel Olcott, Helena Blavatsky, C. W. Leadbeater and others.