The Bette Davis Club (2015) by Jane Lotter.

Or, Margo’s adventures through the money glass.  Reluctantly, Margo goes to the wedding of her niece, and the fun begins in Hollywood.  The spoiled bride bolts, and the vampire mother of the bride makes Margo an offer she of near bankruptcy cannot refuse and a black AMEX card!  She throws in the keys to a red MG and the groom!   Who knew such cards existed? Not us plebs.
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Off they go down Route 66 meeting all kinds of people, some of the most memorable at the Lesbian dance contest in Palm Springs, others with whom they exchange insults over breakfast, a kindly woman who presses a marriage manual on Margo, and Boone who seems destined to be in recovery from head injuries. He should have stuck to football.
Cary Grant even puts in a cameo appearance,. This book has it all, and more!
Finding the bride, after all this, is anti-climatic.  She is a brat.
Margo’s confession at the meeting set a new standard.  Indeed.  No spoiler here. Find out for yourself.
Along the way the imbecilic nature both of Hollywood and its audiences are noted.  
Margo is wrong about almost everything but soldiers on. She may be broke but it is not from a lack of effort.
It shifts gears from silly to serious and back several times but the mix is well judged.  
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I was very disappointed to learn it is a once-off.