In some Beethoven works (e.g. 8th symphony - first movement) there are some parts with repeated sforzando. Are these intended as simple sforzando or sforzando within crescendo? Is there any historical source explaining this?
I'm actually not sure by what do you mean by "repeated sforzando". I guess you are referring to passages that are repetitive and that have frequent sforzandi (for example, a sforzando each bar, etc.).
In any case, there are no fixed standard rules for these things. You have to see the context in which it happens; in general, any interpretation will make sense if it is coherent with the context. See where the passage goes, where does it come from, what is the role in the structure of the music, what is the harmony doing, the character of the music...
It is difficult to generalize. However, Beethoven's music is known for its insistence. How to "insist"? You have to see. Listen to good recordings and try yourself. If the passages in question are steady in rythm, harmony etc. there may be no need to do crescendo. Some people whant to always do crescendo/diminuendo; well, truth is that music can sometimes stay flat. For example, people who is stubborn do/say the same thing many times no matter what; translated to music, a "stubborn" expression will not do crescendo/diminuendo, but just repeat the same thing many times (until at the end it will explode, or maybe exhaust, who knows).
In addition, Beethoven wrote in a quite meticulous way, so chances are that there is no crescendo if he didn't wrote it. Again, this is not always the case and nothing can be generalized. At the end, to bring a score alive, many things that are not explicitly written need to happen.
The sforzando symbol, sfz, is used to indicate a single accented note, much like the > mark, rather than a global change in dynamic level. As such, I don't see what the issue would be with repeating it.
As wikipedia explains:
So in the example below, the off-beat sixteenths are meant to be accented:
To the second part of your question, while I'm sure there are period descriptions of dynamic markings, I am not sure what they are.