I'm a foreigner who has never studied an English grammar book before and who now has the necessity to not only study the topic, but to know everything a good speaker should know. For this reason I would like to know what the most comprehensive grammar of English is.
The most comprehensive grammar of the English language that I am aware of is The Cambridge Grammar of the English Language by Rodney Huddleston and Geoffrey K. Pullum (Cambridge University Press, 2002). This doorstop of a book is 1860 pages long and I strongly advise against using this book as a resource for learning the grammar of English as a foreign language. This grammar is aimed at linguists, not at language learners.
Language learners should look at alternatives such as the following:
- Raymond Murphy: English Grammar in Use. Fifth edition. Cambridge University Press, 2019. This book is aimed at intermediate learners (levels B1-B2) and contains many examples and exercises. A version with an answer key is also available. Due to the level it is aimed at, it is obviously not "complete".
- Marting Hewings: Advanced Grammar in Use. Third edition. Cambridge University Press, 2013. This book is aimed at levels C1-C2.
- Evelyn P. Altenberg and Robert M. Vago: English Grammar: Understanding the Basics. Cambridge University Press, 2010. (281 pages) According to the publisher, "This handy introduction covers all the basics of the subject, using a simple and straightforward style. Students will find the book's step-by-step approach easy to follow and be encouraged by its non-technical language." The book has three parts. Part 1 focuses on "kinds of words", with chapters on nouns, verbs, determiners, adjectives, prepositions, etc. Part 2 focuses on "kinds of phrases", with chapters on prepostional phrases, verb phrases, auxiliary phrases, etcetera. Part 3 is about sententes.
- Michael Swan: Practical English Usage. Fourth edition. Oxford University Press, 2016. This book is organised as an A-Z dictionary of problem points.
- Geoffrey Leech, Jan Svartvik: A Communicative Grammar of English. 3rd edition. Pearson, 2003. (possibly out of print now)
- If you really insist on comprehensiveness, there is Cambridge Grammar of English: A Comprehensive Guide by Ronald Carter and Michael McCarthy (Cambridge University Press, 2006. This book contains over 970 pages (including the index); it does not contain exercises. It is aimed at very advanced learners and people who want to become teachers of English. It discusses many aspects of English grammar that you won't be able to find in the other books in this list.
I absolutely agree that the Cambridge Grammar of the English Language by Huddleston and Pullum is not meant for people learning English as a second or other language. The only one of the "thick" grammar books that is aimed at least in part at learners is Swan's Practical English Usage. But learners probably don't need a "big" grammar at all; one of the (rare) compact grammars such as Rossiter's Descriptive Grammar of English is more appropriate for most learners.