I'm looking for a comprehensive list, or library, that lists a wide range of cryptocurrencies and there exponents/significant digits/decimals - ie "8 for BTC, 18 for ETH, ..."
Information regarding the precision of cryptocurrencies can be found in their respective developer documentation. For Bitcoin, the precision is measured in Satoshis (1/100,000,000 of a Bitcoin) and is the smallest unit of measurement. For Ethereum, the precision is measured in Wei (1/100,000,000,000 of an Ether). For ERC20 tokens on the Ethereum network, the decimal precision is specified by the token and varies.