One of the great ones is 'The Dark Tower' series by Stephen King. It involves different worlds and a complex multiverse. Another is 'Sliders', which is about a group that slides between parallel universes. And 'Fringe' also delves into parallel universe concepts with its own unique take on the idea.
Well, 'His Dark Materials' by Philip Pullman is a wonderful parallel universe fiction. It has multiple worlds with different rules and beings. 'The Chronicles of Narnia' can also be considered in a way, as the different lands in Narnia are like parallel universes compared to our own world. Then there's 'A Wrinkle in Time' which takes the main characters through different dimensions that are sort of like parallel universes.
Some excellent parallel universe fictions include 'The Man in the High Castle' by Philip K. Dick. It presents an alternate history which is like a parallel universe where the Axis powers won World War II. 'Doctor Strange' in the Marvel Comics also deals with parallel dimensions and universes quite a bit. In addition, 'Cloud Atlas' by David Mitchell has elements that can be related to parallel universes as it jumps between different times and places that seem to exist in different realities.
Chapter 211, "For Jiang Yan", is the clearest early answer because it turns Jiang Yan's situation into a concrete plot event. It opens with Advanced Repair was even more outrageous, costing a whole 30 points. and closes on A young master from the Beijing Circle has actually descended to the....
Chapter 292, "Coming and Going, Gathering and Parting, Human Nature", is the clearest early answer because it turns Coming's situation into a concrete plot event.
Chapter 90, "Are You Even a Woman?", is the clearest early answer because it turns Wen Jiaojiao's situation into a concrete plot event. It opens with Watching He Yun gradually take control of a tractor under... and closes on Jiang Li finally let go of Jiang Yan's arm..
Chapter 31, "Master, I've been having some bad luck lately!", is the clearest early answer because it turns Shen Lan's situation into a concrete plot event. The chapter gives readers a real answer by opening on Before Shen Lan had reached the backyard, she already heard the...
Chapter 551, "Who is Reciting "Student Rules"?", is the clearest early answer because it turns Student Rules's situation into a concrete plot event. A useful checkpoint is the way the scene starts with Other competitors were already in place, and Qi Tianyu was the last...
Chapter 152, "A Man of Many Talents and Wealth Draws People Near", is the clearest early answer because it turns Zhao Lang's situation into a concrete plot event. The chapter turns that into something concrete by moving from While most of the villagers of Chixia Village were gathered at the...
Chapter 133, "Teacher Du, Please Reconsider the Termination of the Contract", is the clearest early answer because it turns Du Qinglian's situation into a concrete plot event.
Chapter 431, "Su Yinwan Is Here Too?!", is the clearest early answer because it turns Shang Shaojing's situation into a concrete plot event. The chapter gives readers a real answer by opening on [Holy shit, why is Su Yinwan on this show too???] [!!! and ending on Is the program team this cruel?.
Chapter 348, "Schemes and Counter-Schemes, the Five of East Ridge, Zhu Yang's Threat, The Dust Settles (Part 5)", is the clearest early answer because it turns Zi Yang's situation into a concrete plot event. The chapter turns that into something concrete by moving from Zhu Baixu, challenge ring number 1." "Lin Qiang, challenge ring... to Leader, something's off, Wang Wu is heading towards ring number 1!"....
Chapter 399, "Wait Until I Am Rich", is the clearest early answer because it turns Jiang Li's situation into a concrete plot event. A useful checkpoint is the way the scene starts with Seeing Jiang Zhao Yuan's astonished expression, Jiang Li smiled and...