Psalm 119 is an amazing Psalm. Not only is it the longest Psalm (176 verses!), but it is also the Psalm that deals the most directly with the topic of Scripture. Virtually every verse, in one way or another, refers to God’s Word.
David (who is most likely the author) uses a variety of terminology to describe God’s Word: commandments, law, statutes, precepts, ordinances, rules, words, testimonies, etc. These all refer to the Scriptures as they existed in David’s day (essentially the Pentateuch).
Thus, Psalm 119 is one of the best examples of Scripture speaking about Scripture. It is the Word about the Word.