If you understand German you can check out the „Zedler Universallexicon“. It was published between 1732 and 1754 (a couple decades before the Encyclopædia Britannica) and it’s absolutely massive (68 volumes, about 63,000 pages).
It has been completely digitised (mostly for research purposes, since it is often used to determine the state of knowledge about a certain topic at the time) and is available for free online (http://www.zedler-lexikon.de). Browsing it can be very interesting and often quite amusing.
The entry for California (nowadays „Kalifornien“ in German, but „California“ in the encyclopaedia) is actually pretty long, somewhat hilarious and somewhat sad. On the issue of whether California is an island it teaches the controversy and presents both viewpoints, though clearly seems to prefer the view that California is an island.
I will try to translate the entry quite literally (paragraph breaks are inserted for better readability):
“California, also called Noua Albion [sic, should probably be Nova Albion?], an island in northern America, situated in the South Sea, reaches to New Mexico or New Granada, and is separated from them by a sea arm; however, many believe it to only be a peninsula that is connected to the mainland towards the north. In length it measures 600 or 700 French miles from north to south, from the capes Cabo blanc, Cabo S. Sebastian and Cabo Mendocino to the Cabo de S. Lucar.
The inhabitants are upstanding people, the men walk naked, the women, however, are covered to the knees in skins or feathers from the birds. The people are very skilled in hunting and fishing, as they have a special way of catching the fish.
The land is arid, barren and cold [huh?], even though it is situated where it should rather be hot or at least temperate. There are frequently grasshoppers. Pearls are found eastward on the coasts of California, just as on the coasts of New Mexico and New Granada, as first discovered by Cortesio [Hernán Cortés, I assume] in the year 1535.”
There is also an entry in a supplement volume that expands on the pearl fishing:
“California, island, [reference to previous volume]. Because the coasts there are famous for pearl fishing, the Europeans strongly wished to be there, and also, from time to time, attempted to remain there. In the beginning of the year 1703 a fortress was already built there [I wonder which fortress that is and whether it still exists in some form], so that it can serve to protect the Spaniards in an emergency. It stands in the quarter [not sure about this one, both in the German original and the translation] St Dionysii, on land that the Indians [„Indianer” in the original] call Coneho [Conejo?]. It was given the name of our dear Lady of Loretto.”