Can every book be turned into a GBbook?

No, not necessarily. That is the kind of assumption that sounds appealing but breaks down quickly once you consider format, source material, length, layout constraints, readability, and technical practicality. Some books or texts adapt naturally to the GBbook format. Others may be too long, badly formatted (we’ll do our best to enhance formatting), difficult to source properly, or simply unsuitable for a readable and satisfying cartridge-based experience; if there are lots of special characters, they will take up more space. The longest book I could put in a cartridge was The Count of Monte Cristo, which has 464,641 words in the English translation, so if your book has fewer words, it should fit safely. So the right answer is not “everything is possible,” but “many things are possible, and some need to be evaluated honestly.” That is a better answer because it protects both the product quality and the customer’s expectations.