A.C. Quintero has written a series of Spanish graded readers for beginners. Most of the books focus on adventures beyond the everyday norm, which may appeal to mystery lovers.
Each book’s language level is not clearly identified, but there seem to be four main books for beginners that gradually increase in difficulty: Las Apariencias Engañan (300 unique words), El Escape (350 unique words), El Armario (400 unique words), and Las Sombras (500 Unique words). Two other books (Las apariencias engañan and El Último Viaje) have unidentified reading levels, but you can read a sample on Amazon to see if they are appropriate for you.
A.C. Quintero’s books seem to be effective at helping you review basic concepts, and each book adds a manageable number of new words to increase your vocabulary. They probably aren’t as engaging as the Read It! Or ESLC series, which retell famous plots in simplified Spanish, but A.C. Quintero’s storylines seem to carry more emotion and complexity than those of Paco Ardit or Lingo Mastery.