Time Out says
Look around any Tokyo train and you’ll see that the vast majority of locals (well, those still reading printed books) conceal their choice of reading matter via a wrap-around book jackets. Many Japanese simply opt for the no-frills paper book covers offered free-of-charge at booksellers, but more image-conscious readers make a style statement with an upscale version, often made of leather and sometimes artisan-crafted.
Jinbocho Ichinoichi, a ‘zakka’ lifestyle shop inside the Sanseido bookstore in Kanda-Jinbocho, has you covered with a range of stylish options, encompassing printed fabric and handsome real leather versions, as well as more unusual fabrications such as the tatami used in traditional Japanese floor mats.
Books Sanseido, 1-1 Kanda-Jinbocho, Chiyoda