Every ETH transaction which is created is first added to a pool of pending transactions, from which then the miners pick them. I wonder whether there is any source (research paper / ETH stats website / etc.) or a way to figure out how many ETH transactions are added to the pool per time period (either day/minute/second)?
There are statistics for that. Here is one: https://etherscan.io/chart/tx
Basically you just record how many different transactions your node hears of. All transactions are broadcast to all nodes so you should hear of all of them - at least in theory.
As an extra detail: there is no explicit "pool" of transactions. It's just an abstract construct which means the amount of transactions which nodes are aware of but haven't been mined yet.