This is not picky, but it may be difficult to find really *great* books that fit all of these criteria. First, my fantasy book web site.
Included is a list of only the best sorcery fantasy books, grade reading level, a short review of each book along with a brief summary of adult content. There is also a "Readers' Choice" list of favorite books, and a list of heroine sorcery fantasy books.
As for satisfying your criteria, I recommend these from the list above:
"A Spell for Chameleon" - although not exactly in another world, it's not exactly in ours, either. Plenty of teen romance, lots of action, and set in the late 70s or early 80s (when it was written). I would say that it fills the mutant requirement - though these mutants are the result of too high a concentration of magic, not radiation. The first half of the story is the main character's attempt to talk with someone who goes to great pains to remain inaccessible, though not exactly hidden. Oh, BTW - it also happens to be hilarious.
"Thomas Covenant" - although the majority of events occur in "another world", the main character is firmly convinced throughout the story that he is hallucinating and/or dreaming. He makes frequent returns to the "real" world. What's interesting about this is that the main character has *absolutely* no consideration for the denizens of this "other world" who desperately need his help. This leads to some difficult situations (especially the rape in the first book), but also makes the story a very interesting contrast to Tolkien. In Tolkien, all of the good guys nobly gather together to heroically stave off evil. In Donaldson, the only good guy with the power to stave off evil literally does not care if evil wins or not. In many cases, he is forced to accompany the heroic beings in their quest against evil - and he doesn't much care, as it is all a hallucination anyway. There is little romance, but it is set in modern times (again, late 70s or early 80s), and the main character has major concerns and problems in the "real world". There is little romance, no mutants (well, except for the evil mutant giant triplets...), and no one making any particular effort to remain hidden. There is also adult content, not exactly explicit, but the rape, especially, is disturbing (as rape should be).
I will also recommend "Mrs. Frisby and the Rats of NIMH". This didn't make my list because it lacks magic, but it is an *excellent* fantasy adventure. Set in the mid-70s, the main characters are animals (sorry - no mutants, unless you count the scientifically-enhanced intelligence of certain animals a mutation...). Although primarily concerned with adult animals (particularly Mrs. Frisby), children are significant in the story. Nearly every animal in the story makes a continuous effort to stay hidden - from the humans, the cat, the owl, etc. etc. Overall, an excellent adventure - even though I'm 46.
What your criteria *most* reminds me of is the old "X-Men" or the more recent "New Mutants" comic books. These fit your criteria *precisely*, but being comic books are somewhat less than excellent in literary quality (he says sarcastically). However, if you want mindless adventure in our world in the present day with mutant teens who are always concerned about keeping their identities secret, then this is where I would look. They even have romance (well, I guess you could call it that. I'm not certain what they're like today).
I hope this helps.
· 1 decade ago