Stephen Krashen has published over 500 articles (540 at last count) and a dozen scholarly books in the fields of literacy, language acquisition, neurolinguistics, and bilingual education.  Many of these publications are available for free download at sdkrashen.com and some are posted at researchgate.net. He no longer writes books because nobody can afford them. Nevertheless, he is the most frequently cited scholar in the field of language education.
He is best known for the Comprehension Hypothesis, the idea that we do not acquire language by speaking or writing, or by studying grammar rules. We acquire language when we understand what we hear and what we read, when we get “comprehensible input.” The ability to acquire language in this way remains strong our entire lives.