Essayrater.com checkup

I'm far from perfect writing in English. I'm prone to make wordy sentences, mix up 'its' and 'it's', and other beginner mistakes.
So now that the first draft of my book is done, I've purchased one month access to essayrater.com. It's a site mainly aimed at students writing papers. It gives spelling and grammar advice on about 10 categories. I found the tool:

• Funny: the first chapters of my book had a 70% plagiarism rating. Yes, the tool searches for plagiarism too. Guess what: I was copying the same text that I had posted on this very blog already. I'm stealing from my own...
• Helpful: It got the few its and it's and shelve and other mistakes out
• Insightful: I didn't realise I so often start sentences with 'but' or 'and' or use the passive voice so much. That's a learning point.
• Irritating: If you copy/paste a text from Google Docs, it pastes some HTML markup code in as well, and obviously all that HTML counts as spelling and grammar errors. That's just irritating
  
• Irrelevant: Some categories of what the tool examines are irrelevant for my book. The tool is for helping students write good formal papers. I intentionally use a very informal tone with lots of 'you' in the text. The book should read like a conversation we are having, not a formal lecture.
• Intense: I never realized how much effort goes into text revising. Pfff. My eyes were sore afterwards.

I do hope that effort got the text good enough for the proof readers (thanks guys!) to have a nice reading and concentrate on the content rather than on my spelling errors.