Other resources

It's true that everyone learns the same way when it comes to mastery, fluency, exposure, and real world experience, but it's not true that everyone learns the same way when it comes to deliberate study. Sakubi is deliberate study, and it might not be right for everyone. If Sakubi doesn't start working after two weeks, try one of these resources.

Just make sure you don't kill yourself trying to master them. Mastery only comes from real world experience, and in the case of language learning, that means reading and listening.

Free/Online Grammar Guides and Resources

Tae Kim's Japanese Grammar Guide. Sakubi is inspired by Tae Kim's guide. Tae Kim has vocabulary lists and a built-in mouseover word lookup system. Sakubi expects the reader to have a browser addon installed that does that.
Do not use Tae Kim's "Complete Guide to Japanese", it's incomplete and badly structured. Written in a mix of relatively normal english and "written english". (Use this link if the direct link goes down. Again, do not use the complete guide, use the grammar guide.)
kWhazit's japanese reference. Not a proper guide. Some of the explanations for things like verb forms are more intuitive than other free online resources. Can't be used alone, because it's not a proper guide. Written in a mix of relatively normal english and "written english".
Imabi, a long and almost comprehensive grammar guide made by a student (graduate?) of japanese linguistics over the course of several years. Sakubi's name is a friendly allusion to Imabi. Lesson quality varies dramatically depending on how old the lesson is and how many times it's been edited. As of mid 2017, Imabi's beginner lessons are being rewritten, so if you need more elaborate or contextualized explanations, there's no reason to avoid it. The depth of information in Imabi is, as a whole, far greater than any other japanese grammar resource available legally for free. Written in a mix of "technical english" and "written english".
Maggie Sensei, a grammar resource structured as a blog. I don't know a lot about this but people post articles with good explanations of concepts that other resources present in confusing ways, including Sakubi. Can't be used alone, because it's not a proper guide. Written in relatively normal english.

Physical Grammar Guides

If you live in an underdeveloped country where these resources are not available legally on the first-sale market, some of them are widely pirated. Don't use grammar guides for JLPT levels (N1, N2, etc), they are poison if tests are not your only goal.

A Dictionary of Japanese Grammar: Basic, Intermediate, and Advanced. The "bible" among learner's references for japanese grammar. Note that the entire series is relatively basic, and linguistically complex constructions are not necessarily covered, even if they're learned at a young age by native speakers. In other words, "advanced" refers to being advanced for learners, not for speakers. Aside from that, it's a very good resource, very well established, and proven to be useful. Can't be used alone, because it's not a proper guide. Written in "written english".
Making Sense of Japanese: What the Textbooks Don't Tell You (also known as Gone Fishin'). One of the most brilliant resources to grace learners. Not a proper guide, more like an anti-confusion resource. Very highly recommended, even if you're using Sakubi. Can't be used alone, because it's not a proper guide. Written in beautiful literary english.
A Handbook of Japanese Grammar Patterns for Teachers and Learners (The long-awaited English edition). Basically an alternative to A Dictionary of Japanese Grammar, but more modern. Written in "technical english".

Grammar Videos

Video resources are extremely good for people who don't have a high level of functional literacy in their native language's written form. Sakubi is written in a way that tries to ease in the "written english" feeling slowly over time, but that's not good enough for everyone, especially not for people who only know english in its spoken form and as a secondary language.

Visualizing Japanese Grammar, videos made by a native japanese linguist to explain how different elements of japanese grammar work in the heads of native japanese speakers. Designed for normal people, not linguistics students. Also good in that it doesn't treat traditional grammar, which is bad, as gospel.
Let's Learn Japanese Basic 1 and Let's Learn Japanese Basic 2 by The Japan Foundation. Also known as Yan and the Japanese People. Structured like some kind of graded watcher, switching back and forth between exposure, explanation, and practice. Surprisingly good, and follows real language learning science.
Learn Basic Japanese Grammar 1 and Learn Basic Japanese Grammar 2 from Nihonogonomori on Youtube, by Steve. Structured like a summarized video textbook. Don't bother with any of their other material.