I read this book as a teenager so I remember only a few bits and pieces, sorry. So here goes:

There's sort of 2 main characters: a girl who lives with her mom and younger sister whose family owns an old house by a lake. The second main character is a boy who lives right next to the old house by the lake.

The old house by the lake contains a mystery which both the boy and girl are trying to figure out. The girl sneaks out one night to visit the house (i think). She ends up in the neighbor boy's bedroom to hide or something, and the boy has to keep her under his bed to whole night.

I remember something about there being a lake monster and in the end you find out that the lake monster is actually the girl's father or uncle (some relative).

I can't remember the title but I do remember it being a long one, something like 5 to 7 words. And it was one of those "How to...", "A guide to...", or something along the lines of those types of titles.

PLEASE HELP ME!