For plugin authors

Bookshelf has a few small, stable ways for other plugins (and KOReader user patches) to add to it. All go through ui.bookshelf.

Shelf sources

Put your own books on a shelf: a server's catalogue, a store's library, anything that isn't files Bookshelf walks. The reader adds a shelf, picks your source under Source / grouping, and it behaves like any other shelf.

local function register(ui)
    local bs = ui and ui.bookshelf
    if not (bs and bs.registerSource and (bs.SOURCE_API or 0) >= 1) then
        return false
    end
    return bs:registerSource("komga", MySpec)
end

Bookshelf's own Kindle and Kobo library shelves use the same interface. The full guide is SOURCE_API.md.

Tokens

registerTokens(id, defs) adds tokens of your own to every template: the top panel, cover captions and list lines. The spec is at Tokens.register in lib/bookshelf_tokens.lua.

Cover placeholders

registerPlaceholder(id, fn) draws the card for books without a cover, in place of Bookshelf's lettering. The contract, and how a user patch registers before the plugin has loaded, are at the top of lib/bookshelf_placeholders.lua.

OPDS book actions

registerRecordActions(id, fn) adds buttons to an online catalogue book's download view. The contract is at the top of lib/bookshelf_record_actions.lua.

Micro-modules

A micro-module is one small Lua file with a render(ctx) function. Drop it into koreader/settings/bookshelf/micromodules/ to have it survive updates; a broken one is skipped rather than breaking the menu. The contract is in micromodules/README.md, and the modules that ship with Bookshelf are good examples.

Other book formats

Book formats another plugin registers with KOReader show on shelves like any other book. Bookshelf doesn't fetch covers for them itself, since that can mean a network request per book; a cover the file browser has already made still shows.