Complex sentence: It is a study of the Polish immigrants to America and their families based on personal documents (primarily, letters) as well as on documents such as brochures, newspaper articles, parish and court documents, and so on.
Split into simpler sentences: It is a study of Polish immigrants and their families based on personal documents. The book is a study of Polish immigrants to America and their families based on personal documents (primarily, letters) as well as on documents such as brochures, newspaper articles, parish and court documents, and so on.