[ssn]

Three reference textbooks are available openly:

* Introduction to Information Retrieval, http://informationretrieval.org/

* Information Retrieval in Practice, http://www.search-engines-book.com/

* Entity-Oriented Search, https://eos-book.org/

Modern Information Retrieval is also a classic reference. Not openly available but some contents are (were?) available online. Their site seems to be down but the Internet Archive has a copy.

Additional resources here:

* https://nlp.stanford.edu/IR-book/information-retrieval.html http://web.archive.org/web/20220708135205/http://grupoweb.up...

[firebones]

I am biased, but building the Intro to Information Retrieval chapters in your favorite language, bit by bit, is really good to get the feel of the tradeoffs for index capabilities.