Long Short-Term Memory, Sepp Hochreiter, Jürgen Schmidhuber, 1997Neural Computation, Vol. 9 (MIT Press)DOI: 10.1162/neco.1997.9.8.1735 - The foundational paper introducing Long Short-Term Memory (LSTM) networks, which GRUs simplify.
Deep Learning, Ian Goodfellow, Yoshua Bengio, and Aaron Courville, 2016 (MIT Press) - A comprehensive textbook with detailed explanations of recurrent neural networks, LSTMs, and GRUs.