Four-times married Cynthia had spent the afternoon at Waterstones signing copies of the book, which reveals the secrets of her time with Lennon.
The book charts their relationship through its highs and lows including the day she walked into to find her husband sat with Yoko Ono and realised they were having an affair.
Cynthia now lives with her husband Noel in Spain, where she writes and paints.
Her spokesman said: "She's having an absolutely delightful time. It's lovely for her to be back in the Cavern.
"She's noticing the changes that have been made but when she walked in the door she said: "The design over the door is mine".
"She was very pleased to see that was still there."