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In the book "Quantum Information and Quantum Computation" by Nielsen and Chuang, they use a font which is undoubtedly Erhardt (judging by the regular text), but the italic text seems to be somewhat different from the examples available at major foundry sites. See "ehrhardt-nielsen.png" and "ehrhardt-monotype.png" attached which contain the example from the book, and the same text set with Monotype Ehrhardt. The book variant is less stretched and, in my opinion, is significantly more readable in long italic paragraphs. Does anyone know what subtype is this?
Parts of the book are available on Google Books (not in particularly good quality though), if you need larger samples of text.