David Crystal books in order
9 books in the catalogue, about 60 hours of reading in all; the shortest takes 3 hours and the longest 11 hours.
9 standalone books in the catalogue.
Standalone books
Linguistics 4.9 h 
The Cambridge Encyclopedia of Language 8.7 h 
Rediscover Grammar 4.4 h 
The English Language 5.6 h 
The Cambridge Encyclopedia of the English Language 9.0 h Rediscover Grammar with David Crystal 4.4 h 
English as a Global Language 2.8 h 
The Stories of English 10.7 h 
How Language Works 9.3 h