While author Rick Riordan’s focus is currently aimed at children/teenagers, that doesn’t mean they’re off limits for adults. If you’re a fan of action/adventure and different types of mythology, you’re in for a treat! The characters are well developed and have great senses of humor despite the trying circumstances.
If you love Greek or Roman mythology, start with the Percy Jackson and the Olympians series (first book: The Lightning Thief). For Egyptian, give the Kane Chronicles a shot (first book: The Red Pyramid). Have a fix for Norse? Riordan has recently started on that with Magnus Chase and the Sword of Summer!