It's hard to determine exactly who wrote the first fictional book. As mentioned before, 'The Epic of Gilgamesh' is one of the earliest, and it was written by ancient Sumerian and Akkadian scribes, but their individual identities are not known precisely.
One of the contenders for the first fictional book is 'The Epic of Gilgamesh'. It's an ancient Mesopotamian epic poem. It contains elements of heroism, friendship, and the search for immortality, which are common fictional themes. The story follows Gilgamesh, a king, on his adventures and his relationship with his friend Enkidu.
Since we don't have more specific information about this 'first book of the saxon stories', it's hard to say for sure. But often books like this are a blend. If it aims to educate about the Saxons, it will have a strong historical basis. But in order to bring the story to life and make it interesting, fictional elements like character emotions, and some minor plot twists are usually added.
'Shantaram' is a fictional book. It has fictional characters and a fictional storyline. It may have some elements that seem real - life - inspired, but overall it's a work of fiction.
It's fiction. The story in 'The Shack' is made up by the author. It contains elements like a special place (the shack) and interactions that are more in the realm of imagination. It might touch on real - life themes like faith and relationships, but the overall story is fictional.