Well, perhaps there are stories in 'the book of giant stories' that talk about the origins of giants. How they came to be, what their society was like in the beginning. There could also be stories of giant heroes and their great deeds, for example, a giant who saved his kind from a great disaster.
The 'book of giant stories' might contain stories about the relationships between giants and nature. Giants are often depicted as large and powerful beings. So, there could be stories of how they shape the land, or how they are in harmony with certain natural elements. It could also include stories about giant families, their traditions, and how they pass on their knowledge from one generation to another.
There could be many. Maybe stories about giant adventures, like their battles or their quests. It might also have tales of how giants interact with normal humans.