A) 2.2M {{cite books}} in 1.1M articles
We know for certain there are 1.1M articles containing a {{cite
book}} and assume 2x per article on average.
B) 4.4M book citations overall
We might assume for every templated book there is a non-templated
thus 2.2M x 2
C) 300K links can be added pointing to archive.org
IABot fixed ~ 100K books. Noisbn might fix another 100K. Another 100K
require manual (this may be an overestimate).
D) 100K unique books on Wikipedia can be linked to the Internet Archive
If there is a 3:1 ratio of links to unique books then
step C / 3 = 100K
E) 1.46M unique books on Enwiki
Assuming 4.4 million book citations and 3:1 ratio of unique books
4.4/3 = 1.46M
F) 1.36M unique books on Enwiki that don't exist at the Internet Archive
1.46M - 100K (step D)