Micromastery is the practical aspect of polymathy. When you believe that it is normal to be interested in many things you naturally seek an efficient way to learn. I have found the efficient way to learn very much helped through an interest and immersion in the current projection of Sufi thought in the West. And the Sufi tradition has always drawn attention to the need and naturalness of being polymathic. Sayings can degenerate over time: the correct phrase is NOT 'Jack of all trades and master of none' but, "BETTER to be a jack of all trades THAN a master of none." We've all met the master of none- he is the person pontificating that there's no point in doing something because conditions/tools/time aren't perfect. He is the person hiding his laziness behind the concept of specialisation and assumed mastery. Actually specialists are very limited beings unless they refresh themselves at the well of more various interests and pursuits. Hence the need for a polymathic approach even if you work in a narrow field. And if the field is so narrow that it cannot ever develop then that sounds a little too narrow to me.
But the idea for polymathy, though attractive to me since childhood really came from immersion in modern work about Sufism. Such a book has just been republished: Sufi Thought and Action by Idries Shah (though it includes other authors too). The publisher is ISF publishing.