To be frank, you are going to have a tough time reconciling "advanced history" and "reading online", at least not unless you have access to online services like JSTOR or ProQuest. Your public library may have a subscription, at least if you are in a bigger municipality, and many states allow access to the library at the state universities as long as you are a resident, so that is also an option. In any case, what you are looking for for "advanced history" is going to be peer reviewed articles in journals, or books from university and other academic presses.

Our booklist has a good selection for World War II, which you can look to, but if you want larger options, one good way is to get a general work, Gerhard Weinberg's is a good one, and check the bibliography and footnotes to see what they are citing a lot for the area(s) you are interested in, and drill into that more specific work.

Another way is to find Comps reading lists which are often put online by universities. They often will be broader than WWII, but ones like this have large sections which do focus on the conflict.

As I said though, these aren't websites. You'll need to borrow from the library or buy these books.

Might I recommend that what you actually want to read in order to read and understand advanced history is historiography? I'd recommend people like Carl Becker, Hayden White, March Bloch, and others. I'd recommend starting with the Very Short introduction, That Noble Dream and the Block book. Understanding these will help you understand what you mean by advanced history and the limits of the things you're reading now.