Britney Spears Thanks Fans For 'Woman In Me' Memoir Success, Says It's Breaking Records

Britney Spears is over the moon over the spectacular success of her tell-all memoir, "The Woman in Me" ... and she jumped online to thank her millions of fans for their help in breaking records.

The singer hopped on Instagram Tuesday night -- the same day her book dropped -- to share her excitement ... "My book is the highest selling celebrity memoir in history and it’s only day 1 !!!"

She went on to thank the fans for their support, with good reason ... the book's sitting atop both Amazon and Barnes and Noble's best sellers lists since it debuted. It hasn't shown up on the New York Times Best Sellers List ... at least not yet.

As you know, Brit left a lot of juicy details about her past on the pages of her memoir ... dishing about her rocky relationship with Justin Timberlake -- claiming she had an abortion at his behest.

She also talked about her conservatorship, left a glowing review of her ex, Sam Asghari, and wrote about how her former hubby Jason Alexander "got sh**faced" before getting married in 2004 -- something Jason told us was total BS.