It's a problem with a long tail, and it very much depends on what objective you're optimizing for. In search at least, you aim for "good" and "better", but will never achieve "perfect". It's a pretty interesting space at the meeting point of software and data science. You probably don't necessarily need to read full books before diving in, but play around with "learning to rank" https://xgboost.readthedocs.io/en/latest/tutorials/learning_... and maybe check out https://www.microsoft.com/en-us/research/uploads/prod/2017/0... . Also https://www.tensorflow.org/recommenders/examples/basic_retri... .