The original Toaru Majutsu no Index light novels were adapted as anime and manga. I heard, that the LNs had much more content than the manga or the anime, but I couldn't find any reliable sources. Do the light novels have more content than the manga/anime? And if yes, do the adaptions differ much or are the differences minimal?
I've read the first ten novels from the series and I'd say the content is pretty much the same, i.e. the anime follows the novels pretty well, but of course, the novels are much more detailed, for example more explanations during the fights, small additional scenes or more detailed explanations on certain (pseudo-scientific) concepts regarding the story. Also, in the novel you can often read what the character is thinking at the moment, which gives certain scenes a different feel than in the anime.
Personally, I found some parts of the novel rather streched-out, but on the other side, some other scenes were really fun to read. If you have time and you really like the anime, you should check out the novels, but don't expect a blast.