The In-universe How to Speak Dragonese appears to be a leather-bound notebook created by Hiccup. He gathered all his information on Dragonese - the language of the dragons - into this notebook, and he carries it around with him everywhere. He is quite proud of it.
In this book Hiccup kept notes on the Dragonese language and descriptions of the various species of dragons and their habits.
— Book 3
In Book 3, Hiccup carries How to Speak Dragonese with him during the 'Boarding-an-enemy-ship' lesson, aboard the The Hopeful Puffin. Snotlout rams his boat into Hiccup's and sends it whirling off course. Hiccup and Fishlegs find themselves near the Summer Current. In his fear and in the fog, Fishlegs ends up boarding a Roman ship. He is carptured and Hiccup saves him. As they are leaving the Roman ship, the Thin Prefect grabs the How to Speak Dragonese notebook and rips it in two. He also captures Toothless. Hiccup is devastated at the loss of his dragon and destruction of his prized notebook.
Later in Book 3, after being captured by the Romans, the Thin Prefect takes the other half of the notebook from Hiccup. When Hiccup is put in an arena to fight on Saturn's Day Saturday, Hiccup enlists the help of Ziggerastica and a multitude of tiny nanodragons. Under their power, Hiccup seemingly flies out of the arena and claims to be the Norse God Thor. He demands the Romans leave the Barbaric Archipelago and to give back the tattered notebook. The Thin Prefect had repaired the notebook, "the two halves sewn together carefully with golden Roman thread."