aforementioned work; op. cit.; cited work
Formal academic expression used in footnotes, endnotes, and bibliographic references to refer to a previously cited work.
For details, please refer to the aforementioned work.
Op. cit., p. 32.
同書 also means 'the same book' or 'the aforementioned book', but is more general and can be used when the work was just mentioned, not necessarily formally cited earlier.
Compound of 前 (before, previous) + 掲 (put up, display) + 書 (book, document). The combination 前掲 means 'aforementioned' or 'cited above', and adding 書 specifies a written work.