ReadEdenHow to read the Remembrance of Earth’s Past trilogy
The core trilogy
Read Cixin Liu's The Three-Body Problem, The Dark Forest and Death's End in that order. Each book continues the situation created by the previous one. Starting later would remove both context and discovery.
Ball Lightning is separate
Ball Lightning was written before the trilogy and shares some scientific and thematic territory. It is not volume zero and is not required before The Three-Body Problem. Read it independently once you know you want more Liu.
The Redemption of Time has a different author
The Redemption of Time was written by Baoshu, not Cixin Liu. It began as fan fiction and was later published with authorisation. It responds to the trilogy but is not its fourth volume in the same authorial sense.
The practical route
Finish the three Liu novels first. Then decide whether you want a separate novel by Liu or another writer's extension. Keeping publication and authorship visible prevents a large fictional universe from becoming a misleading numbered list.