I never read the introductions to novels first and a good example why is Pankaj Mishra's intro to JG Farrell's The Siege of Krishnapur (which by the way is a very highly recommended book). Mishra does offer a little bit of analysis and spends about a page situating the book within a tradition of Mutiny novels, an entire subgenre I didn't even know existed. (I place that to being American but suspect that even most Brits aren't aware of it either.) But Mishra spends most of the introduction on a plot synopsis that even gives away the ending. Why? Either we're about to read the novel and don't want this or we're coming back afterwards and don't need it. I'm picking on this because it's just something I've read recently but it's not an uncommon problem with novel introductions.
But what really got me started is David Rieff's introduction to the current edition of Graham Greene's The Lawless Roads, his account of a 1938 trip to Mexico. Rieff is mainly, actually almost exclusively, concerned with the book as a precursor to The Power and the Glory and as such he finds it wanting. He attacks the travel genre as "narcisstic" and only a "miscellany" even stating that the novel is more universal. This seems an odd way to introduce a travel book (which he does claim is "very good") even if you think it's true. And I suspect he would have been even harsher if the book is considered journalism or reportage.
What's most clearly missing in Rieff's introduction is, well, everything. Why did Greene write the book? The back cover says it was a commission but for whom? Did they want a conventional travel account or a report on religious suppression? (They didn't exactly get either.) The copyright page reveals that it was published in the US the same year as England (1939) though under a different title (Another Mexico) but was it published in Mexico? What was the reception? How accurate is Greene's account? What did he leave out? Has anybody identified the unnamed but very specific people Greene encountered? Did Greene have any opinions about the book at publication or later?
There's also a suggestions for further reading included but everything is related to Greene's life. Nothing about Mexico or its history. In fact even considered as purely non-historical the book desperately needs notes. There are references to Campion and Peguy (Greene didn't bother with first names) that I got, or at least think I got, but there are numerous mentions of other things (an unspecified Jules Romains novel, bits of British culture, Mexican politics) that I and most readers would need help deciphering.