RISC-V is an open source ARM-like CPU design standard originally developed at UC Berkeley that may do for CPU hardware what Linux did for operating systems.
RISC-V International has grown from 29 members in 2015 to 2000+ members today.
the RISC-V exchange lists available boards, cores, software and books. Unfortunately, the links to books is broken as of August 23, 2021.
David Patterson, the father or RISC, is the vice-chair of the RISC-V Foundation. He is also known for two popular books, Computer Architecture, a Quantitative Approach, and Computer Organization and Design, both authored with John Hennessy.