There seems to be some confusion about the enumeration books. Those are the books we are familiar with from 1841 to 1901, but the Enumerators' Summary Books for 1911 are quite different. They are the books that the enumerators used to list all the household they had enumerated, listing each address, the name of the head of household, and the numbers of males and females in each household. But they also described the type of building, whether it was purely residential, or retail premises, for example. This was very useful if your shopkeeper ancestor lived 'over the shop' or if their business and home addresses were separate. Not only was the Enumerator's Summary Book the only place that described building use, it was the only place that listed vacant properties, and non-residential premises like churches, factories, workshops etc.
In earlier censuses at least some of this information would be recorded in the enumeration books. So the fact that there are only household schedules, and no summary books, for 1921 is most unfortunate, because it means there is no information on uninhabited buildings. But at least we will have the Plans of Division, which is some compensation. They describe the boundaries and contents of the enumeration districts (which was also in the 1911 summary books) and we don't have anything like that for the 1939 Register which makes it very difficult to work out the location of any particular enumeration district, and absolute nightmare in rural areas
If anyone wants to see what the various schedules, books and instructions for 1921 and other census years, there are lots of examples on HISTPOP
http://histpop.org/ohpr/servlet/Browse?path=Browse/TNA%20Census%20-%20Other%20(by%20date)&active=yes&treestate=contract&titlepos=0