Only good book I have on the Eastern Front is Paul Carell's "Hitler Moves East, 1941-1943".
As for the Dora (i.e.: 80 cm K (E) railway siege guns), it was pretty much a waste of resources. Yes, it was a technological marvel, capable of hurling a 15,000-lbs. shell up to 23 miles, but it was more economical and effective to simply send several aircraft the 23 miles to drop a 1000kg bomb, with far less resources. The only advantage it had was that it couldn't be shot down en route to the target. Other than that, another example of squandered German resources with little return on investment, similar to the Maus and E-100 super-tanks.