Built-in Functions - open(), Python Software Foundation, 2023 - Official documentation for the open() built-in function, detailing its various modes (including 'a' for append) and how to interact with files in Python.
Python Crash Course, 3rd Edition, Eric Matthes, 2023 (No Starch Press) - An accessible introduction to file handling in Python, providing practical examples of reading from, writing to, and appending data to files, suitable for beginners.
Learning Python, 5th Edition, Mark Lutz, 2013 (O'Reilly Media) - A comprehensive guide to core Python features, offering detailed explanations of file input/output operations, different file modes, and proper file management techniques.