No real recommendations this time round, as I have not read any of the following books myself, so cannot – strictly speaking – recommend them.
But since these books or e-books all cover the topic of “hidden film gems”, I assume that you might find them interesting, so I add these here as “options”:
1) Leonard Maltin’s 151 Best Movies You’ve Never Seen
2) Robert K. Elder: The Best Film You’ve Never Seen – 35 Directors Champion the Forgotten or Critically Savaged Movies They Love
3) Rupert Alistair: Classic Movies – 14 Films You May Not Have Seen, But Should
4) Rupert Alistair: Classic Movie Gems – 16 MORE Films You May Not Have Seen, But Should
As I said, I have not yet read any of them, but Leonard Maltin’s expertise is well-known, while Elder’s book has received positive reviews from journalists and fellow critics alike. Also, all items on this brief list have received positive reviews on amazon.