In a scene in Love, Simon, Firestar's Quest can be seen at the top of a stack of objects near his desk.
James Patterson's Witch & Wizard series, in the back of each book, features descriptions of books, celebrities, etc. banned by the characters' government, which are all parodies of real-life books and people. There is one parodying Warriors:
THE BRAWLERS: The story of a pack of sentient dogs - some stray, some pets - seeking to fulfull a 'prophecy'.
In the graphic novel of Coraline, there's one panel toward the start where it says "She'd already read all her books", and it shows a stack of books. One is the cover of the first Warriors book, Into the Wild.
Live Action TV
In the Pretty Little Liars episode "Run, Ali, Run", Aria remarks; "A's been stalking you for two years like a warrior cat..."
In the The Middle episode "The Math Class", right at the start of the episode, Frankie dumps Brick's backpack out onto the counter. The Darkest Hour can be seen on the top of the pile.
In the Law & Order: Special Victims Unit episode "Surrending Noah", Long Shadows can be seen on Olivia Benson's bookshelf near the end of the episode at the 39:07 mark.
In the We Bare Bears episode "The Kitty", the bears adopt a "cat". When deciding on a name for it, one of Grizzly's suggestions is "Tigerclaw", and Panda replies, "Nah, too evil", referencing the Big Bad of Warriors.