It is possible, but it depends on the story
As other answers have noted, a book needs to end properly. The exceptions are arbitrarily-divided books like Lord of the Rings, or books from a series where you choose to use cliffhangers to link the volumes.
How to end a book: what you need to do is to have an ending that works as such, but with a new story that, in-universe, starts literally right after the ending scene closed. For example:
Faeria was now safe, and the Chosen One could finally rest. Once an awkward teenager who didn't know how to fit in his life was now a mature young man. Trials and hardships, friendship, love and loss, so much had happened to him... Alas, as much as he loved the lands of Faeria and its denizens, he knew he could not stay - as he belonged to Earth. After tearful farewells, he walked through the portal.
As weeks became months, and months became a year, she had lost hope for her missing son to ever be found again. A small part of her, a mother instinct, had refused to give up, and even now she would not wear the black of mourning. When she heard steps, and saw a broad-shouldered silhouette, she thought her elder son had come back to retrieve one of the documents he so often forgot. Then tears blurred her vision - she had recognized him. Her little boy had grown so much!
"Hello mom. I- I'm home."
And then the second book starts with:
A sound echoed across the city, like had never been heard on Earth, heralding sorrow and loss - the call of a White Wyrm. As it burst in this unsuspecting world, he had felt another tear in reality - something else had come from Faeria! It was now flying with all its strength, for it knew what were the stakes. If the knowledge of Faeria was passed on those humans, if they learned the weakness of the Wyrm, the invasion itself would be endangered.
What happens here is that the first story ends - the hero, now changed and mature, returns home. The second story is completely different, as it is about Earth defending against an invasion. But the invasion itself happens to start right at the hero comes home, because (say) dimensional portals to Earth only open at a very specific time, once every who knows how many years.
In a way, it's a coincidence that the two events happen to be causally linked to the same thing, and thus happen at the same time. The difficulty is to craft a believable coincidence, otherwise it may feel contrived.