If current Wikipedia is printed in a book form, it will span about 2000 books, each with 1000 A4 size pages and still not including diagrams, images and equatio

If current Wikipedia is printed in a book form, it will span about 2000 books, each with 1000 A4 size pages and still not including diagrams, images and equations. But Wikipedia is extremely shallow. A PHD typically requires study of 250 to 400 textbooks starting from Kindergarden (these are strictly course requirements). If you assume there are about 10,000 sub-sub fields of expertise for human kind (everything from sub-sub field in genetics to sub-sub field in airplane design) I think there may be about 1 million textbooks currently in print that covers pretty much every core expertise. But again textbooks are not deep enough either. There are 1.3 million papers/research articles that gets published every year. A conservative estimate of all research articles ever published might be 50 million. That would be about about 250,000 printed books, each about 1000 pages. I think we can comfortably say that most of the core human knowledge can be fit in to about 1 to 2 million printed books, each about of size 1000 pages.