Artbook:
https://store.steampowered.com/app/2116830/The_Art_of_Borrowhelen/
OST on Steam:
https://store.steampowered.com/app/2116820/Borrowhelen_Soundtrack/
Also Borrowhelen Deluxe Edition bundle is available.
This bundle type is 'complete the set', so it's a good deal even if you already own Borrowheln.
https://store.steampowered.com/bundle/27480/Borrowhelen_Deluxe_Edition/