Bism Illah wa assalaamu alaykum wa Rahmatullahi wa Barakatuh.
Many people assume that living in Saudi Arabia, and particularly Madinah, means that all the books available here are Islamically correct and compliant. However, that is far from the case. There are wonderful collections of ahadeeth and Islamic texts that are all used by the Jami'ah students and scholars for reference, but there are many others that are incorrect, unchecked, and deviant.You either need to know your books, so you can purchase those that will truly be beneficial knowledge, or ask someone to advise you what to buy. That said, Islamic books are well worth buying here - particularly if you are buying Arabic ones or from Darussalam - as they are cheaper and the selection is exhaustive maashaa Allah.
When it comes to English books and fiction, it is quite a different story. There are Arabic translations of the totally unsuitable and un-Islamic Twilight series, Harry Potter, and more. The same books, as well as others dealing with Western families with divorce, boyfriends, and various other typical subjects, are available in English. In addition, there majority of books have pictures, images, photos, and can even be of Christian influence. We have seen children's Islamic books that talk about Muslim children doing ballet, having Christmas decorations, and celebrating secular holidays. It is not easy to find a good variety of non-fiction books for children and young adults here and many of the school and story books have spelling and/or grammar errors.
Should you want to order books, the place we use the most is The Book Depository. They ship worldwide for free and we have always received our shipments, maashaa Allah. You can find a very good collection of books there, usually cheaper than buying them here, and can select books more suited to your needs. Of course, there is always Amazon.com as well, but you have to pay shipping for the books, which can be very expensive.
Barak Allahu feekum.