The main themes in the novel and primarily in Chapter X are metafiction and metatheater.
Metafiction, according to Patricia Waugh, is:
fictional writing which self-consciously and systematically draws attention to its status as an artifact in order to pose questions about the relationship between fiction and reality. In proving a critique of their own methods of construction, such writings not only examine the fundamental structures of narrative fiction, they also explore the possible fictionality of the world outside the literary fictional text.
Metatheater is, according to Lionel Abel:
...reflecting comedy and tragedy at the same time, where the audience can laugh at the protagonist while feeling empathetic simultaneously.
In Part II, Don Quixote, lives in the world of literature (the romances he reads), so much so that we have an author's note from a fictional author, Cide Hamete Benengeli, who wants us to skip this chapter!
When the author of this great history comes to relate what is set down in this chapter he says he would have preferred to pass it over in silence, fearing it would not be believed, because here Don Quixote's madness reaches the confines of the greatest that can be conceived, and even goes a couple of bowshots beyond the greatest.
So, we have an author (Cervantes) presenting an author (Cide Hamete Benengeli) presenting a character (Don Quixote) who thinks he is another character (a knight errant), which is both comedic and tragic to the point that his perfectly sane sidekick (Sancho) is pretending he sees a character (Dulcinea), who doesn't exist, but is really another character (a smelly peasant woman), all simply to indulge Don Quixote and--most importantly--us! In this world, as it says below, everyone wins! In other words, the world of literature is the world to live by.
One critic's take is as follows:
The point of Don Quixote's knight-errantry is to make a fantasy come true. Living a fantasy even for a short time is more than most hidalgos could say. His friends unwittingly bring his wish to fruition better than he could have possibly hoped. Everyone wins, for "what the world needed most of all was plenty of knights-errant" and by acting in his fantasy, his friends help revive the traditions of knight-errantry.
In fact, it is their indulgence—their cooperation with the fantasy—that fulfills Don Quixote's dream and "astonished [him], and for the first time he felt thoroughly convinced that he was a knight-errant in fact and not in imagination." Don Quixote's madness, sadly, is the only way for adults to play in the serious world of Spain's Golden Age.