Errors are not limited to e-books. I recall way back in college a Professor required use of a text book he had written for a course in Catalytic Chemistry; first edition published by Wiley. So the first day of class he hands out, I kid you not, 30 pages typed of errata. Some minor stuff yes, but also chemical equations that did not balance and significant missing content; as in “as illustrated in example 5.1 above.” No, not even there. I was forewarned by students who had taken the class in previous years about this so it was not like it had just been discovered in a newly published work. I am certain that the publisher was waiting to sell out the first run before republishing a corrected edition.

Anyway to get back to the subject. If the original book was only available in paper form and scanning with OCR was used to get the electronic version errors are bound to happen. I have been a volunteer proof reader for Gutenberg Project and I know how much proofing and review goes on there before books are released as final. Even so errors can still be found in texts downloaded from there. I don't know how much proofing Google Books does for the books they have scanned, the public domain ones free for complete download, but these tend to have a lot of errors.