From what I've read it appears that the concrete undermined the structural integrity of the forts at Liège. Also the enormous German guns were inches larger than the designer of the forts had prepared for.
So, if those German guns knocked out the Belgian forts in relatively short order, how did the fortresses at Verdun survive the battle? Were they just newer?
Any suggestions for books on the battle of Verdun would also be appreciated.
The Forts at Verdun, if I'm right, simply were not subjected to the sheer level of firepower that the Liege forts were; the situations were also significantly different.
At Liege, the forts were bombarded by 21, 30.5 and 42 cm pieces, at fairly short range, and in a shorter period of time. Moreover, by the time the forts at Liege were being reduced, the Germans had already outflanked the position. If the forts didn't fall by force, then the garrisons could be starved out/forced to surrender.
At Verdun, the French could run supplies to the forts, keeping the garrisons in action. At Verdun, the French had artillery supporting the forts, shooting BACK at the Germans, whereas the defenders of Liege were very much on their own once the Germans invested the city. The forts at Verdun, I believe, were also somewhat stronger than those at Liege, which were built to withstand 21cm fire, but not much over that. Verdun was also a SIGNIFICANTLY larger operation, and the German artillery had a lot more targets to cover, over a wider area, than at Liege.
Paul Jankowski's Verdun is good; Alistair Horne's The Price of Glory is still the classic with regards to Verdun. For the French Army in WWI in general, I couldn't recommend Robert Doughty's "Pyrrhic Victory" highly enough! Also try Robert Foley's " German Strategy and the Path to Verdun: Erich von Falkenhayn and the Development of Attrition, 1870–1916". Steer clear of John Mosier's "Myth of the Great War".
Hope that helps!