Basically the only thing you have to do in Japanese is find the book and put the book in the cart (green button that says カートに入れる). Then go to the cart (カート, shopping cart-shaped button toward the top right) - once you go into the cart, there a button at the top right for language, so you can switch the checkout/payment screens to English. If you don’t have an account already, the account creation/sign-in pages can be switched to English too, it seems.
In terms of finding the book to read (once you’ve bought it), you just have to click the gear button (マイページ) at the top right (then you can switch to English again and go to My Library -> find book and hit この本を読む).
I think I’m actually going to try e-reading for this one too :o
Because the thought of adding another physical book to my already-overflowing bookshelf of unread Japanese (and English) books is giving me weird anxiety ahaha I’m never going to finish all of these ><