So I don't speak Spanish (very well, anyway), but I've read many books on Chernobyl in English, some of which exist in translation. The book I'd currently recommend to people in English (Plokhy's Chernobyl) doesn't seem to be in Spanish yet (it came out last year). Grigori Medvedev's The Truth about Chernobyl (La verdad sobre Chernobil) is, however — it's pretty good on the account of the accident, the consequences, the technical stuff. Not as good as the social/political as Plokhy. But even most English-language books aren't as good as Plokhy's.

For a more "human" feel, I might pair Medvedev's book with Svetlana Alexievich's Voices of Chernobyl (Voces de Chernóbil), which is an oral history of Chernobyl and its aftermath. You have to take such things with a grain of salt (more of a grain of salt than the HBO people did, unfortunately), because people's memories and experiences are not always strictly what happened, but it's still valuable.

If Plokhy's book ever becomes available in translation, it's what I'd recommend first — he manages to weave together a much more comprehensive story than anyone else has so far, and it's very well-written (in English, anyway).