The Redemption of Time is Baoshu's continuation of Liu Cixin's Remembrance of Earth's Past trilogy, written with Liu's blessing after the release of Death's End. In his own foreword, Baoshu is explicit that he does not claim the book as official canon — he describes it as one dedicated fan's attempt to fill gaps left by the original trilogy.
The novel follows Yun Tianming, a supporting character from the earlier books, after his consciousness is intercepted by the Trisolaran fleet. Held and interrogated for decades, he is drawn into the aftermath of the Trisolaran crisis and, eventually, into a conflict operating on a far larger, more abstract scale than the original trilogy's war between Earth and Trisolaris — one concerned with the fate of the universe's dimensional structure itself.
Readers who finished Death's End will recognize the returning characters and settings; this is a continuation, not a starting point for the series.