The RISC-V Foundation publish the official specification.

• Online viewer: https://riscv.org/specifications/
• GitHub source: https://github.com/riscv/riscv-isa-manual

I find HTML easier as a reference. These are un-official conversions to HTML from the GitHub source:

• Latest User ISA
• Latest Privileged ISA

ABI References:

• Standard ABI: https://github.com/riscv/riscv-elf-psabi-doc/blob/master/riscv-elf.md
• Embeddded EABI: https://github.com/riscv/riscv-eabi-spec/blob/master/EABI.adoc

Other Resources:

• A “green card” summary of the ISA:
  • RISCVGreenCardv8-20151013.pdf
• A machine readable set of opcodes is available on GitHub: https://github.com/riscv/riscv-opcodes