Miley Cyrus & Justin Bieber's Moms -- Power Lunch in L.A.

It was a meeting of the power moms -- Miley Cyrus and Justin Bieber's mothers on a lunch date in L.A. today ... and Justin's mother was an OPEN BOOK.

We can only imagine what they talked about -- money, money, and more money -- but one thing's for sure, Mama Biebs ... Pattie Mallette didn't hesitate to pass along a copy of her new autobiography "Nowhere But Up." (Foreword by Justin Bieber ... of course)

Tish Cyrus left the cafe with a conveniently-posed copy of the book tucked under her arms.

As for other interesting talking points -- Tish said there's still no date for Miley's wedding to fiance Liam Hemsworth.