Is the statement that prohibiting setting out on a journey to visit the Noble Grave necessitates prohibiting setting out on a journey for a book fair correct, and what is the correct Islamic response to this issue, and is a dual intention (for the mosque and the grave) permissible when setting out on a journey?

The gist of the answer:

The prohibition of undertaking journeys (شد الرحال) to shrines, graves, and mosques, with the exception of the three mosques, does not conflict with the permissibility of traveling for other legitimate purposes such as trade or seeking knowledge. The noble Hadith: "Journeys are not undertaken (لا تُشَدُّ الرِّحَالُ) except to three mosques" pertains to the prohibition of traveling to a place for the purpose of venerating the place itself, such as traveling to visit a mosque other than the three mosques, or the grave of a prophet or a saint. This type of travel is what the prohibition applies to, because it involves veneration and reverence for these places for their own sake. As for traveling for other purposes, such as trade, jihad, seeking knowledge, or maintaining kinship ties, it does not fall under this prohibition, because the purpose is not to venerate the place itself, but rather to fulfill a need or achieve a benefit.