After that is the Euphorbia Journal, and a subscription (and of course, all back copies) to Euphorbia World.
I would recommend a copy of Hermann Jacobsen's "Lexicon of Succulent Plants." Although it deals with other succulents, and was published 40 years ago, it's the basis for modern Euphorbia classification (except the Madagascans), and mostly makes sense. There's a 3-volume "Handbook...." but it doesn't really add anything, at least in terms of Euphorbias.
After that it's harder and more expensive! Harder because you're either dealing with individual articles in journals, or the antiquarian/botanical book market.
If you get into the latter, be aware that prices vary! For example, I just ordered a copy of Susan Carter's "Flora of Tropical East Africa, pt. 2 Euphorbiaceae" from a UK vendor for 32 GBP, as opposed to the $80 that it's going for on Amazon
It's from 1988 so not that antiquarian, but there ya go!
-R
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