Malamander by Thomas Taylor
Malamander is a middle grade novel set in a fictional seaside town or indeterminate era. The tale is told from the point of view of Herbert Lemon, a boy who has the job of managing lost and found items in a hotel. Except, being the sort of town this is, the role comes with a little more mystery and adventure than you might expect. Herbie, or Herbie to his friends, must search for the family of a girl who believes herself to be lost. Throw in a mad watery adversary and a sea monster, and we have everything necessary for an adventure.
Where Thomas Taylor really succeeds is in giving the story heart. As he notes in a podcast interview, a key aspect of finishing this novel was to pare down the word count, ensuring every phrase matters. And that clean approach lets the characters shine.
Thomas is working on the sequels, and I would recommend that you get the first if you haven’t already.