de

"de" always takes the ABLATIVE. Its literal sense is "down from", which is the one to hold onto, because the figurative sense grows straight out of it: a book is "de amicitia", down from the subject of friendship, hence "about" or "concerning". Roman book titles are full of it.

That second sense is where most of your reading will meet it, and it is the ancestor of the "de" and "di" of French, Spanish and Italian. Do not confuse it with "ex" (out of, from inside) or "ab" (away from the outside of): all three take the ablative, but they point in different directions.