Python literature relevant to QGIS

The PyQGIS cookbook is a great resource

https://docs.qgis.org/latest/en/docs/pyqgis_developer_cookbook/

Original: This is coming soon: The PyQGIS Programmer's Guide: http://locatepress.com/ppg

Update 2019: There is a second edition out now: http://locatepress.com/ppg3

While this book is not specifically QGIS and Python, it was written for open source coding using GDAL and OGR with Python: http://www.packtpub.com/python-geospatial-development/book