Start spell-checking what you write. Now.
I recently read a very interesting book entitled Amazing Jellies: Jewels of the Sea, by Elizabeth Gowell. I bought this book from the Aquarium in Atlanta last summer, after visiting the jelly fish* they have there, and being utterly mesmerized. They are so beautiful.
I read the short book in a couple of hours, and I really liked it: the facts are fascinating, the photos are beautiful, and the content is detailed just enough to introduce these creatures to someone who doesn't know much about them, like me.
However, there is one thing that turned me off: typos.
The book is not full of them, but there are quite a few typos here and there. Also, a scientific word that was new to me was spelled in two different ways throughout the book, and I had to look up the right way to spell it.
This is not good. Besides displaying the wrong information (wrong scientific spelling), it is quite annoying to run into typos in a book. Misspelled words are not acceptable in a book. If you call yourself an author and you work with a publisher to make a product that people actually buy, your publisher has to have it double- and triple-checked by professionals. Typos reflect badly on you and your publisher, and unfortunately, they could make the content of your (non-fiction) book less credible: when a publisher does not bother having their product quality-checked, chances are they also do not review the content they are sharing, or verify their sources.
This is not always true, but it could definitely be viewed this way. I can't help but take an author who lets typos slip, a little less seriously.
If the spelling and grammar in a book are not flawless, it doesn't necessarily mean that the content has no value. Likewise, if the spelling is perfect, it doesn't necessarily mean that the information shared is all true. But that's a different topic.
Truthfulness aside, I think that spelling and grammar should be as important to the author and the publisher as the content itself. Especially if the product is not free.
Bad spelling, bad grammar, and typos are a turn-off and they should be taken seriously.
If you run into any typos or mistakes in my blog, please let me know. Although English is not my first language, and I don't call myself an author, I want my writing to be correct, always. I have all my posts checked, if I can, before publishing them, and you should too. ☮
* As per the book mentioned, scientists don't like the term "jelly fish" because these creatures are not fish at all; scientists prefer to call them jellies.