An introduction to Japanese Syntax, Grammar & Language is a large, online introduction to Japanese as a whole. While the title says "introduction," this is really more than just a simple introduction. The author goes into much detail about Japanese grammar, words, particles, sentence patterns, conjugations, phrases, and more. It seems like there was also a paperback version available on Amazon, though it unfortunately says "out-of-print" as of the time of this write-up.
As it covers basic concepts of Japanese grammar in a single webpage rather than in a database format, searching for specific grammar points can be a little bit of a challenge. However, since this site truly starts from the beginning of learning Japanese, it could be used as an online Japanese textbook. It must be noted, however, that, like many of the other potential textbook-replacer grammar guides, it does not contain exercises or self-assessments.
This is a great resource for beginners to deep-dive into the nitty-gritty of Japanese grammar. For that reason, compared to other online grammar guides, it might not cover as many idiomatic grammar patterns, especially intermediate to advanced-level expressions.