Can't believe something like that creates bad impression in tech materials, but I have always been surprised with myself about how my impression changed due to grammatical mistakes.
I wouldn't avoid the book because of that, but I don't know about others. Such mistakes might make one think the author is uneducated, sloppy, or both. In a free ebook and with english as a second language (guessing that is the case), it's totally excusable.
It has to do with attention to detail, and for me it raises the possibility that I can't trust the technical aspects of the writing. For sure that's a grammar-bigotry on my part, I'm not proud, but I simply look askance a bit after seeing that kind of thing.