Than, as now, armies prepared plans to cover different eventualities and revised them on a yearly basis.
Zuber also produced an excellent book called "The Real German War Plan 1904 – 1914" in which he set out in detail the German War Plan for each of those years and how it was revised in accordance with normal practice to meet the ever changing political and military situation.
Schlieffen did indeed produce a plan when he was Chief of Staff but he retired in 1905 and there was no obligation to stop the annual revision after that. Certainly the German General Staff did not feel such an obligation. The German army went to war under the latest 1914 revision of the War Plan.
However the idea has taken root that his was THE plan and that any subsequent revision was a weakening of the purity of his plan and its guarantee of success.
The German war plans contained two elements – a concentration plan for the armies – and – an operational plan or mission for each of those armies with a timetable so as to co-ordinate their movements.
Although this sounds good it effectively took operational control out of the hands of the Commander in Chief who, in the early stages at least, was reduced to resolving disputes between army commanders or giving in to the whims of some of the more aristocratic army commanders on the left wing by allowing them to attack when the plan called for them to defend. The German army had good army commanders but they were not good subordinates.
Plan XVII on the other hand was not an operational plan but rather a plan of concentration of the armies and was sufficiently flexible to allow Joffre to make changes up to the last minute. Once in place the armies awaited orders from Joffre who had complete control over all operational matters. Joffre demanded obedience and dismissed army commanders who did not carry out his orders. He also dismissed many Corps and Divisional commanders as well.
I have not read the Siege of Liege book yet but I agree with Ponder above that Zuber is not an easy read.
Regards