"I thought, 'Are you kidding me? I can't believe she's so insensitive,'" recalls Garth, whose 11-year marriage to actor Peter Facinelli had ended earlier that year.
The reality of that symbolic wake-up call (now displayed on her living room coffee table) was just what the actress, who's also a mom to daughters Lola, 11, and Fiona, 7, needed to move on with her life and rebuild her career.
The former Beverly Hills, 90210 star penned Deep Thoughts From a Hollywood Blonde, available this coming Tuesday.
"It's not a salacious tell-all," Garth tells PEOPLE of her memoir, which deftly balances humor, honesty, poignancy and a touch of behind-the-scenes insight from her 90210 days. "It's my journey and an introspective look at things I've experienced in my life."
Garth is also renovating the Los Angeles area home she shares with her daughters for an upcoming HGTV series, The Jennie Garth Project.
"It feels good to be busy and be creative," Garth says. "I feel like I'm fulfilling my destiny now in a way that I never have."
For more from our exclusive interview, pick up the latest issue of PEOPLE, on newsstands now.