"The Summer Muskie" was published in 1976 and describes how a good muskie guide figured out how to catch muskies during the warmest summer days when nobody else could catch anything.
Before I summarize the book we first need to understand lake stratification. Here is the Wikipedia article on lake stratification, and the better description on one of its parts: a thermocline.
First lets describe lake stratification, or turnover. The temperature of a body of water varies with its depth. The sun heats the surface water faster than the bottom, and cold winter temperatures cool the water temperature faster at the top than the bottom. Colder water is denser than warmer water. Denser water sinks to the bottom. The water's temperature varies throughout the season and so the layers of water change with the season. The water's top layer will be separated from its bottom layer by a middle layer, known as a thermocline. The cold, dense layer at the bottom will also have less oxygen than higher layers. Note also that clearer waters will change temperature faster than darker waters.
The reason that all this is important to fishing is because fish, including muskies, are cold-blooded. Muskies in particular seem to be most active at a temperature around 65 degrees Fahrenheit.
In the spring, fish will spend their time in shallow water because that is the water that is warming, and getting closer to their preferred temperature.
Fish will move throughout the lake in late spring and early summer.
When the temperatures of the water change to the point that the lake stratifies, the fish will also change their location in the water.
On to the book.
In Tony's book he points out that muskies tend to like living in the thermocline, because the water is too warm above, and too cool and oxygen deprived below. He also points out that, at least in Northern Wisconsin, a lake's thermocline will at be around 25 ft. This also means that lakes that do not get 25 feet deep will not stratify.
"No matter how far from the structure a muskie may suspend, it will suspend in relation to the structure."
He illustrates in his book that fish prefer structure to no structure. If there is structure in the thermocline, then that is where the fish will be. If the structure is above, or below, the thermocline, then the fish will be as close to the structure as they can be and yet remain in the thermocline.
The weather plays an important part in muskie fishing. Tony describes good weather for muskies as, "...overcast, partly cloudy, or rainy and a brisk wind makes the conditions even better." Poor weather conditions are bright, sunny and calm.
I'm not going to summarize the whole book, but keep these last few points from it in mind while you fish for suspended muskies:
Muskies "... will not suspend below the thermocline."
Structure is important.
Muskies suspend only in lakes that stratify, when they stratify.
Thermocline, thermocline, thermocline.
Tony recommends using spinner baits first for fishing suspended muskies. His "Rizzo Wiz" is what he will be using usually. He has also been known to put near ridiculously huge weights at the frond of his Wizzes in order to get them down to the thermocline. Other baits might be deep diving crankbaits, like a Cisco Kid or his Rizzo Diver, which is a sinking crank bait. I recently used a large Reef Runner crankbait to catch this thin fish.
Muskies always need to eat and so you should be able to catch them even when they are suspending.