Britney Spears Audiobook to Be Co-Narrated by Her and a Mystery Celeb

Britney Spears' memoir is coming out soon, but only a part of the story is going to be told in her voice when it comes to the audio version -- the rest will be handled by a surprise celeb guest ... TMZ has learned.

Sources connected to the publisher of "The Woman in Me" tell TMZ ... Britney has already recorded the voice-over folks will hear when they buy the audiobook, but we're hearing she only recorded herself narrating a portion.

We don't know how much she recorded, but we've been told she did not narrate any chapters having to do with her family -- and, presumably, much of the book will cover that topic. Our sources say she didn't record audio for those sections because she found them too painful to retell in her own words.

So, as a remedy ... another celeb was tapped to record the rest of 'TWIM' -- which comes out on Oct. 24 -- although it's unclear who was on the mic. Our sources tell us the publisher was really pushing for Reese Witherspoon to pick up the baton, but she wasn't available to do it.

We're told only female celebs were approached, and they eventually landed on someone suitable who's completed the recording ... and the whole audiobook is ready for the masses.

Remember, Britney was on the fence about doing an audio version of the book at all -- something she was unsure of dating back to July.

With that said, it's perhaps not surprising they got her to agree to narrate at least some of it -- audiobooks are a huge part of any new book rollout these days because they're big-time sellers.

Seeing how Britney got paid a fortune for this memoir -- it makes sense Simon & Schuster wanted to get its money's worth by producing the audiobook.