I will try to formulate this question so that it makes sense for a wider audience and suits the style of the QA here. Feel free to suggest changes.
I am a non-native English speaker living in the US and the UK for several years now on and off, and have always performed very well in school with languages. I hold a master's degree and graduated from university a few years ago. This likely applies to many others as well.
Recently, I was offered (and have accepted) representation through a literary agent and will be starting work on a non-fiction book soon. In fact, this will actually be my second book (the first one was very technical and written in my native language; it also had very little narrative). The upcoming book, however, will be published (and edited) in English.
Here's what I am trying to understand and figure out:
In the process of preparing to write longer segments (the pitch and sample chapters were rather short), I am trying to learn as much as possible about writing correctly. Don't get me wrong, I fully understand that writing style and content are the defining characteristics of a great book. However, writing grammatically correct even short segments like this question without embarassing myself in front of the editor has become my biggest fear now.
Is it possible as an educated, grown-up, curious researcher and avid learner to master the art (science) of English grammar completely (so I could fool all of you into thinking I'm a native writer), within a reasonable amount of time? I have recently started to read a lot about core topics like commas, quotations, tenses, and a whole lot of other things randomly. My main concern is lack of understanding when it comes to defining the horizon. When will I know that my grammar is indistinguishable from any native writer? Of all topics at hand, which are the low-hanging fruit? Which are super-hard but really, really necessary to understand? For instance, what parts of this question might give away my non-native origin?
Is there anything that you as a native speaker can identify as classic non-native grammatical mistakes? I feel like starting with eliminating those, progressing onwards to the advanced mistakes (these include mistakes made equally frequent by non-natives and natives), and then simply investing a lot of time in finding style and a voice would be a worthwhile approach.