The joke is that Kobato misremembers the name of the book (車輪の下, Sharin no Shita, Beneath the Wheel) and says some random name that sounds vaguely close (じゃりン子の舌, Jarinko no Shita, lit. Struggling Student's Tongue), but then the book with that random name ends up existing in-universe. But it's not actually real.
I mean true that but that doesn't mean we're still not going to read it. Also it's kinda cute on how kobato is trying so hard to get closer with tsuruda reminds me of my old days 🥹