This is a weird book to read because it's a book on the Loch Ness Monster written fifty years ago. That means that there have been fifty years of development in lake monster hunting. So it's a rather obsolete book to read. So do I rate it base on the validity of Costello's claims through today's light, through 1974's light, or how much enjoyment I got out of it?
I chose the latter. The book was easy enough to read, but I feel that sometimes Costello got bogged down in the wealth of info on lake monsters out there and didn't let you chew on the individual ideas long enough. My favorite sections were the ones on mythology and local oral history, which may also be the most timeless parts of the book. Most of the the ideas presented sound solid to me, but I'm no expert and, honestly, not convinced.
Still, it was an enjoyable enough read and a classic of its arena. If you're interested in Nessie or other lake monsters, give this one a shot.