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bauerhjb ; You can roll your own icons see
Drop-in Icon Replacement Sets
Extensive library of icons here
Browse Icon Packs

Darn : but 3 of those functions (Import an HTML or DOCX file, Add cover and Create new, empty book) use the same image file - book.png, which you can only replace once - sorry for the misinformation.
And for Create checkpoint I see the pushpin icon (same as Mark Books uses in the library GUI).
You don't have to replace all the icons, I've just replaced 2 or 3.
BR