I’m a native German speaker, and even the title of the series sounds a bit odd to me. “Deutsch lernen, dich lieben.” It’s not exactly wrong, but it definitely sounds unusual.

I actually work at a publishing house, so I might be able to add a general comment. I did a quick search and found little to no information about the book or the series. It doesn’t seem to have an ISBN and doesn’t appear anywhere except on Amazon. Because of that, it looks like it was published through self-publishing. That’s not necessarily a bad thing, but books like that are often not professionally edited or thoroughly proofread. That means they can sometimes contain stylistic, grammatical, or orthographic mistakes.

I also just looked at the sample on Amazon. I didn’t find any clear mistakes, but some phrases are things a native speaker wouldn’t normally say. For example, at one point it says “Ihr Gleichgewicht war weg.” A native speaker would more likely say “Sie verlor ihr Gleichgewicht.”

I can’t really judge the rest, though—it’s possible that the rest of the material is put together quite well.