Following on from my earlier grumble about how the catalogue of Life handles literature, I've spent an afternoon mapping publications in the "itis".publications table in a copy of ITIS to external GUIDs, such as DOIs, Handles, and SICIs in JSTOR. The mapping is not complete by any means, but gives an idea of how many publications have GUIDs.You can view the mapping here. Many of the publications in ITIS are books, which don't have DOIs.