A lot that I'd have mentioned have already been brought up - I heartily second both the
Bartimaeus books (as well as Jonathan Stroud's new
Lockwood & Co. - I didn't warm to it after the first book the way I did to
Bartimaeus, but it fits and is well written) and anything by Rick Riordan in particular.
I know I bring this up a lot, but it IS my all-time favourite series (which just got finished - the ninth and final book was released yesterday) and it fits the topic and if a 30-something woman's personal taste may be called in question, it's also Rick Riordan's sons' favourite series.
So, yeah,
Skulduggery Pleasant by Derek Landy.
It starts off as a kids' series, but "grows up" like Harry Potter did, so the final books are very much YA, I'd say. Lots of violence (and I mean a LOT, especially later on); relationships exist and matter, but it's not a series focused on romantic love or romance (love is important, but more the family/friendships way).
Other than that, I'd say 12 - especially if reading on a higher level - is the kind of age where most YA books should already be suitable, at least any that aren't strictly girl-oriented (nothing wrong with the girl-oriented YA, but a lot of them do have a heavier focus on romance and love triangles and beautiful bad boys).
Things like
The Hunger Games should be okay though, I'd imagine, or
Maze Runner, mentioned above (I didn't personally love it but many people do). Also Neal Shusterman's
Unwind series (the last book should be out this year) as well as the
Chaos Walking trilogy by Patrick Ness (starting with
The Knife of Never Letting Go). And the
Leviathan trilogy by Scott Westerfeld should be suitable.
I'd also recommend
Katya's World and
Katya's War by Jonathan Howard (girl protagonist, but absolutely no romance - adventure/action sci-fi set on a future human colony world).
Two books I just read recently -
Itch and
Itch Rocks by Simon Mayo - also excellent "boy" action/adventure thrillers. A bit like
Alex Rider (which also fits as a recommendation here), just with more school and chemistry and less international spy stuff.
Oh! The
H.I.V.E. (Higher Institution of Villainous Education) series by Mark Walden - definitely written for boys in that age group, lots of fun with enough depth to it.
Probably also most things by Eoin Colfer -
Artemis Fowl, his new
W.A.R.P. series. Also, I think basically anything by Brandon Mull would fit the age category and be suitable content-wise.
I've read more that should be fitting, but I'm having trouble remembering them all right now.
Some of the recs above are middle grade (such as Rick Riordan), but I've tried to include only the ones that work well across age groups, i.e. they might be for 9-12-year-olds but are absolutely okay for readers of any age above that.
Out of older stuff, I'd say "pretty much any Jules Verne" and Alexandre Dumas and Walter Scott (
Ivanhoe,
Rob Roy) and Arthur Conan Doyle's Professor Challenger books and... oh, many. 12 is a great age for reading.