Zen-Existentialism, by Lit-Sen Chang, was ultimately disappointing. I learned of it from a reference in the chapter on Buddhism in the book Kingdom of the Cults; the reference suggested that Zen-Existentialism would explain a connection, or certain similarities, between Zen and existentialism. (Note that I didn't say "Zen Buddhism"; that's because Chang indicates in Zen-Existentialism that Zen is so different from Buddhism as to be arguably a different belief system.)
Although you can tease out those connections or similarities through close enough reading, the book largely fails, due to inadequate discussion of existentialism. As Lit-Sen Chang was an ardent Zen practitioner before accepting Jesus Christ, he intimately knew the Zen worldview and teachings, and spends half the book explaining them and comparing them to Christianity. This is certainly a good book from which to hear a Buddhist-turned-Christian explain Buddhism and especially Zen (which, again, he claims is different from Buddhism).
That works well enough, but the second half of the book is basically a tract for Christianity. It has much less to say about existentialism (and how it's similar to Zen) than about Zen (earlier in the book). That is unfortunate, because that's what I read the book to find out; and Jesus Christ is already my Lord, so I didn't particularly need to read a tract.