Monero is an instance of the CryptoNote protocol. There's a brief reference to the difference in privacy properties between CryptoNote and Zcash in section 1.2 of the latest Zcash protocol spec:

"The basis of the privacy properties of Zcash is that when a note is spent, the spender only proves that some commitment for it had been revealed, without revealing which one. This implies that a spent note cannot be linked to the transaction in which it was created. That is, from an adversary’s point of view the set of possibilities for a given note input to a transaction —its note traceability set— includes all previous notes that the adversary does not control or know to have been spent. This contrasts with other proposals for private payment systems, such as CoinJoin [Bitcoin-CoinJoin] or CryptoNote [vanS2014], that are based on mixing of a limited number of transactions and that therefore have smaller note traceability sets."

https://github.com/zcash/zips/blob/master/protocol/protocol.pdf