To add a few comments…
My boyfriend tells me that Wikipedia is particularly good for mathematical and scientific concepts. Entries often go into substantial detail, but since they’re written for a non-specialist audience, they don’t use jargon or make assumptions about prior knowledge. Plus, you can click on terms you don’t know to look at their entries. In fact, in cutting-edge areas, Wikipedia is often more accurate than traditional encyclopedia which take years to publish. I suppose the parallel for history would be cases such as the online version of the DNB (Oxford Dictionary of National Biography) is more current and more frequently updated than the published version.

A strategy I’d recommend is to keep Wikipedia open when reading, and look up locations or people mentioned in the text. It’s great to be able to glance at a map of Florence or a portrait of Pope Pius IV as one reads about them.

Another perk of Wikipedia is that when you encounter a typo, you can just correct it!