Uncommon spells are the 11th through 20th spells of each magical Realm. Uncommon spells include a few mid-tier Fantastic Unit Summoning Spells, but for most realms they largely include low-cost spells that are nonetheless quite effective. Uncommon spells will often be the lynch-pin of a wizard's strategy early in the game, and carefully building one's strategy around the early availability of even one such spell can make a huge impact. Later on in the campaign, Uncommon spells will see a great deal of use, particularly in conjunction with stronger spells (i.e. in support of your increasingly-powerful assets).
Any wizard with at least 1 Spellbook in a magical Realm will be able to research a number of Uncommon spells. The number of such spells available for research is limited by the number of Spellbooks acquired. The exact spells available for research are randomly selected. With 8 or more spellbooks in a single realm, a wizard will be able to research all Uncommon spells of that realm. Uncommon spells require 300 to 800 to learn.
With 11 Spellbooks in a single realm, a wizard can pick 2 Uncommon spells from that realm which will be available as soon as the game begins.