Articles about nominal inflection might also give you information about adjectives. You can look for things like "adjective agreement"-- try to expand your search terms to see what comes up.
If you can't find it elsewhere, you can always find information about general properties of a language in a descriptive grammar. (Note that a descriptive grammar is not a pedagogical grammar [=textbook] and describes things in technical linguistic terms, rather than using shortcuts to help students learn to speak the language.) But the section there will likely be shorter than articles on the topic, and may not go into all of the questions that articles do.
You might also find more information in English than in French, if you're allowed to use non-French sources. Or for that matter in German (and Italian, even Latin, etc.).
There may also be some very useful information in works about the history of French, giving examples of how Latin changed into Old French and then into modern French, with examples of what you're looking for in each section.
