Enid Blyton <br/><br/>
Well Done, Secret Seven 

<br/><br/> The Secret Seven have built a new meeting place, and it's a tree house. But someone else has discovered their hideout, and he's on the run from a gang of thieves. 
<br/><br/><label>ISBN: </label>9780340917565 
<br/><br/><label>Dewey Class. No.: </label>English BLY