After completing the deadric quest "Discerning the Transmundane" you obtain a skill book called Oghma Infinium. This is a skill book and gives +5/+4 to particular skills depending on which path you choose when reading the book.
The book is only allowed to read once (unless a glitch is exploited), but I noticed when I read the book (which was only once) the total number of skills books I have read did not go up. This therefore did not count towards the "Reader" achievement. Why isn't this book considered a skill book?