For theoretical foundations, I'd recommend reading about the FLP result (https://dl.acm.org/doi/pdf/10.1145/3149.214121), the reduction of the state machine replication problem to consensus (https://dl.acm.org/doi/pdf/10.1145/78969.78972) and the different consistency criteria.

To climb on the shoulders of giants you can dive into the work of (incomplete list, irrelevant order but these have been eye openers to me) :

- Leslie Lamport

- Maurice Herlihy

- Eli Gafni

- Rachid Guerraoui

- Michel Raynal

- Petr Kuznetsov

- Hagit Attiya

- Sergio Rajsbaum

- Nancy Lynch