Although summoners might not seem as if they're rooted in mythology and popular fantasy literature, they represent the iconic character that accomplishes much with the aid of a powerful, otherworldly ally (or sometimes slave). Aladdin and his genie, the king and the witch Zoulvisia, the tale of the genie of the bronze ring (from The Blue Fairy Book by Andrew Lang), and even the tall tales of Paul Bunyan and Babe the Big Blue Ox can all serve as templates for how players can play summoners, and how they might be seen by societies within a campaign world.
Because a summoner's eidolon can take many forms, you can use the class to represent a wide range of character types. If the eidolon takes the form of an incubus or succubus, the summoner becomes a diabolist. If it takes the form of a dragon, the summoner becomes a dragon rider. If it takes the form of a primal nature spirit, the summoner is a shaman. If the eidolon is a formless mass of horrific, shapeless flesh, the summoner becomes an alienist working with powers beyond his comprehension.
We could present many more types of summoner and eidolon than any one or two books has room to offer, especially since most game supplements have to appeal to a broad range of interests. The Advanced Player's Guide lacks room to present more than the most basic eidolon abilities, and Ultimate Magic logically must focus on the most popular magic-using classes. The summoner and his eidolon benefit greatly from additional options, each expanding the kinds of characters that can be created from one class, but such options are of necessity interesting only to players of that single class, and they take up a lot of room.
To help in that regard, Advanced Options: Extra Evolutions presents two new base forms (the hut and the ooze) and thirty-eight new evolutions. The evolutions focus on 1- and 2-point evolutions, and many provide additional color that adds to an eidolon's build. With these new options, an eidolon can become a brass elephant with a built-in howdah, a horrific mass of shifting flesh, or a legendary animal spirit that can draw on any of nature's natural defenses.