Surprisingly, I bought this book during the first days of Border's Books and Music closure sale - and what a wonderful reference to have on hand. My jaw about dropped when I saw that Mahler had conducted an all-Wagner concert with the NY Philharmonic at the National Theater here in Washington, DC, a few months before his death. About the only drawback is that it doesn't document Mahler's performances of opera (though the book title says "Concerts"), and I wish it documented these as well. I'm sure that Mahler's performances at the Met can be found rather easily.
On the left page of the open book, one can see a marquee poster announcing the upcoming concert that was conducted by Mahler, and the right page documents the full details of the concert itself. Indexes are very thorough as well.
Wade