Yobgorgle: Mystery Monster of Lake Ontario is a children's novel by Daniel Pinkwater. It is one of his more substantial novels, although my copy is only 113 pages long. It is a light and fun read, quite silly but perhaps a bit less so than many of his other works.
This is the tale of Eugene Winkleman, a young boy sent to live with his uncle while his parents are travelling in Europe. He finds himself in Rochester, New York where he knows no one and has nothing to do. As any young boy would do, he hunts up the local cryptozoologist and gets a summer job helping to hunt the elusive yobgorgle. It is not long before they have enlisted both his uncle and the local multibillionaire to join them in their adventures. I can't tell you much more without giving away spoilers, but suffice to say that they have an exciting and rather silly adventure.
This is perhaps one of Pinkwater's more boring adventures, having only one plot element that is clearly magical, and only moderately amazing scientific devices. It makes up for that by having very odd characters, although he's done odder. It is written in Pinkwater's usual style of simple clarity, making it very readable. This was one of my favorite Pinkwater books when I was a teenager, but I do not think I would recommend it as someone's first experience with Pinkwater's novels; better to go for Borgel or The Snarkout Boys and the Baconburg Horror. Speaking of which, Yobgorgle is currently easiest to locate in the collection 4 Fantastic Novels by Daniel Pinkwater. This volume also includes Borgle, The Worms of Kukumlima, and The Snarkout Boys and the Baconburg Horror.