Search the full text of books that Google has scanned and stored in its digital database. The books come from large research university and public libraries. Full text is available for books published before 1923. For books still under copyright protection, only a small snippet of text is available.
When you find a book that is relevant to your topic, always check the bibliography for other sources and then ask a librarian for help with finding them. The author of the book has already done much of your research for you, and he/she spent many weeks/months/years of his/her life doing research. Take advantage of it!
You may also explore OWHL's collection by youself via NOBLE Catalog to get titles like these:
Republican Beijing: The City and Its History from ebrary, Inc.