Postal Covers From Germany

I have a box of postal covers and postcards mostly from Germany going back to German States thru the Third Reich and a few other countries. I feel a little overwhelmed and don't know where to start. I am trying to get an idea of what they are worth. I have a Scott catalog which helps give me the used stamp value but I don't know anything about postmarks, franking, etc. I read that Michel makes some catalogs but they are quite pricey. Does anyone have any online resources or links?

I am attaching a picture of one of my oldest and best covers. It is postmarked Breslau 30 over 8 in the middle and 8-9 Ab: I believe the stamp is Prussia 1857 Scott #8 3sg yellow. If anyone can tell me more about the postmark or what the cover may be worth, I would appreciate it.

If it`s endeed 1857 Prussia (Preussen), according to a 2005 catalogue it`s between 150-500 Euro on cover. 3 colours: orange, yellow, darker orange: 150, 450, 500 Euro (have no idea that's in $). Of course it's Catalogue Value, and from 2005!
Other info: king Friedrich Wilhelm IV. No watermark.
This type of cancellation (with information regarding the hour resp. evening, between 8 an 9) seems to be used from around 1838.
Ps. I'd like to see nice old Hungarian or Austrian covers, if have any (I know more about them)

Breslau is now Wroclaw in Poland.

And something else. It was reprinted in 1864, in german a so called Neudruck (Nd). And really important: there are many fakes (even covers). Catalogues suggest to be careful.