Coco and The Book of Life both take place on Día de los Muertos, but they're very different movies. There's a phenomenon in Hollywood where two extremely similar movies are released in the same year, or otherwise relatively close together, by separate studios seeming without knowledge of each other - these are called twin films. Although The Book of Life, from animation studio Reel FX and distributed by 20th Century Fox, was released more than three years prior to Disney and Pixar's Coco debuting in theaters, there's been plenty of discussion about whether the two could be twin films, or whether Coco simply ripped off The Book of Life.
The Book of Life follows two childhood best friends, Manolo Sánchez (Diego Luna) and Joaquín Mondragon Jr. (Channing Tatum), who grow up and fight for the love of their other friend, María Posada (Zoe Saldana). However, the gods of the afterlife - La Muerte (Kate del Castillo) of the Land of the Remembered and Xibalba (Ron Perlman) of the Land of the Forgotten - interfere and Manolo is sent on a journey through the afterlife in order to return to his beloved. In Coco, the young Miguel (Anthony Gonzalez) rebels against his family's rule forbidding anyone from playing music and accidentally travels to the Land of the Dead. With the help of Hector (Gael García Bernal), Miguel searches for his musical idol, Ernesto de la Cruz (Benjamin Bratt), the only person he believes can help him get home.
Related: Is Pixar’s Coco Too Scary For Kids?
In terms of their development, Coco director Lee Unkrich reportedly first pitched the concept in 2010 around the time Toy Story 3 came out. However, The Book of Life was in development for much longer, with director Jorge R. Gutierrez toying with the idea as early as 2001. Although Guillermo del Toro was on board as a producer and the film was set up at Reel FX, there wasn't much momentum behind the movie until it was picked up by 20th Century Fox and given a 2014 release date.
So, The Book of Life was undoubtedly being developed long before Coco. But, while Pixar/Unkrich and Reel FX/Gutierrez may have known the other was working on a movie about Día de los Muertos, it's unlikely they each knew specific details about the projects that would become The Book of Life and Coco. Undoubtedly, they would have kept their respective films under wraps until they were ready to reveal details in a public fashion. With that said, in terms of public appearance, Coco wasn't announced as in development by Pixar until 2012, and by the end of that year, The Book of Life had started production. So they seemed to be on the development track at the same time, even though Pixar's long productions mean Coco didn't release until three years after The Book of Life.
Additionally, Coco became entrenched in very public controversy in 2013 when Disney filed an application to the U.S. Patent and Trademark Office for the phrase "Día de los Muertos." Disney was justifiably criticized by the Latino community for attempting to trademark a slice of their culture. While those behind Coco learned from this particular mistake - bringing in Chicano artist Lalo Alcaraz, playwright Octavio Solís, and former C.E.O. of the Mexican Heritage Corp. Marcela Davison Aviles to become the film’s cultural consultant group - there's no doubt that the production of Pixar's latest has left a sour taste in the mouth of many.
Still, while The Book of Life and Coco both tackle themes of family surrounding the holiday of Día de los Muertos - with aesthetically similar styles, using bright colors and staples of Mexican culture as inspiration - they do so in extremely different ways. The Book of Life is a love story that explores the importance of bravery and confronting one's own fears, while Coco is a coming of age tale that's much more rooted in family. Certainly, there are superficial similarities between Coco and The Book of Life, but they offer much different takes on Día de los Muertos and Mexican culture. As such, they're different enough to both be worth a watch (and perhaps a couple rewatches as well).