Long Short-Term Memory, Sepp Hochreiter, Jürgen Schmidhuber, 1997Neural Computation, Vol. 9 (The MIT Press)DOI: 10.1162/neco.1997.9.8.1735 - The original academic paper that introduced the Long Short-Term Memory (LSTM) architecture.
Deep Learning, Ian Goodfellow, Yoshua Bengio, and Aaron Courville, 2016 (MIT Press) - A comprehensive textbook that covers recurrent neural networks, including detailed explanations of LSTMs and GRUs.