'The Woman In Me' is due out this October.
"Sam is thrilled that Britney finally has the opportunity and freedom to speak her truth with her book," the source says." He sees it as an opportunity for Britney to open up about what she has felt the need to address for far too long without being silenced or filtered."
"He is confident that this will be a therapeutic experience and help her let go of many ongoing personal and professional hardships she has faced over the years," the source adds of the 29-year-old actor.
As for how Spears is feeling about the impending release, the source says she's "feeling excited and relieved about releasing her book."
"She feels like a huge weight has been lifted off her shoulders," the source says, which echoes what a source told ET last month.
"She is looking forward to finally voicing her story in such a big way," the previous source said of Spears, 41. "She's not feeling nervous at all and she's actually very eager to get it out there."
When it comes to their romance, the source says that Spears and Asghari, who tied the knot last year, are "doing well," adding that "they are still very in love."
"Sam is very supportive, comforting, and dedicated to Britney. He wants her to be happy and encourages her to be her best self," the source says. "Communication is key in their relationship. Sam is a very active listener and provides Britney with honest, healthy, and constructive advice in an effort to grow together."
The Woman In Me will hit shelves Oct. 24.