Kingdom Hearts Ultimania: The Story Before Kingdom Hearts III https://www.amazon.com/dp/1506725236/ref=cm_sw_r_cp_api_glt_fabc_98QWT69Y8MRG3FXE0176?_encoding=UTF8&psc=1
It would help to know a little about them and how much you want to spend. If they like books, they may enjoy Kingdom Hearts Ultimania which is on sale right now. The Kingdom Hearts Character Files is also a good option.
Kingdom Hearts Ultimania: The Story Before Kingdom Hearts III https://www.amazon.it/dp/1506725236/ref=cm_sw_r_cp_apan_glt_i_7GYKESANQ17QSAJE9YME?_encoding=UTF8&psc=1
https://smile.amazon.com/Kingdom-Hearts-Ultimania-Story-Before/dp/1506725236
Not to say whether there's an answer in there, but we'd finally have a big ol' guide like this officially published in English.