The book series The 100 by Kass Morgan was made into a television show on the CW, and the similarities between the books and the show stop at the title.
The television series uses the same plotline; however, it is sped up and changed. In the show, the officials, and parents of the children, are shown regularly, unlike in the book. The entire four-book series is changed and made into one season. Then, the next 6 seasons are created from scratch.
Not only are the plot lines modified in the television series, but the characters are as well. In the novels, the main characters are Clarke, Bellamy, Wells and Glass. Whereas in the television series, the main characters are Clarke, Finn, Bellamy, Finn, Raven, Jasper, Octavia, and Monte. The show does not include the main characters Wells or Glass from the books. Furthermore, the television series features the parents of the children as main characters. In the books, the parents are barely mentioned or dead.
I personally have not watched the entire series, but I have read the book series. If you were to pick one to read/watch, I would recommend doing both, as they are completely different stories. However, I did enjoy the book series a bit more because it was more detailed and suspenseful.