In the Lone Wolf series of game books by Joe Dever and Gary Chalk, Summerlund was the name of the kingdom that was the initial setting for the books. Sommerlund was a relatively small kingdom in the Northern section of Magnamund, but it plays a very important role in the Battle of Good and Evil, being the major bulwark against the lands of the Darklords, to the west.
Okay, if you can just accept that last sentence for a while, there are a few more things to be said about Sommerlund. It has a generally medieval level of technology, much like the rest of the world of Magnamund, and it is divided into five sections, each ruled by a feudal lord. There are several large cities where commerce goes on, but most of the country is still agricultural. There is a king, whose main job is to keep the Darklords in check. The people are generally blond haired and Northern European, and worship a solar god. The name seems to be some type of dog olde English way of saying "Summer Land", which is yet another sign that these are people on the side of good.
Okay, now that I have gotten through my second paragraph of ridiculous explanation, I can best sum up Summerlund by being a generic fantasy kingdom, with a few somewhat disturbing implications, mostly about the good blond haired English stand-ins being the world's appointed saviors against the forces of Darkness. This is somewhat unfortunate, especially since in other places, the Lone Wolf books are quite innovative and interesting in their fantasy. But the core of the books does seem to be built around high fantasy, and the cliches thereof.