I would suggest some of George Orwell's books...
Animal Farm and
1984 in particular. Even though he is not a 'modern writer,' he was around during Tolkien's day, his books are still a good read.
I also disagree with Rowling's Harry Potter as being 'too childish.' Rowling delved into many of the old myths that Tolkien loved doing. One can have just as many 'intellectual debate' about Rowling's books as they can about Tolkien. She mostly sticks to Roman and Greek myth, as well as French influence, but her knowledge in those areas is quite impressive. Particularly (like Tolkien) with her use of names, and the meanings of names...Albus Dumbledore, Sybil Trelawney, Remus Lupin, Sirius Black, Draco and Lucius Malfoy, Hermione Granger, Minerva McGonagall, Filius Flitwick, Severus Snape, Voldemort, Nicolas Flamel, and the list goes on...all have interesting translations (or are historical figures) that fit with the personality of the character.
As an example, Severus Snape. Severus was the name of 2 Roman empires, and 'snape' means "to be hard upon" to "rebuke" or "snub." Hmm...I say that describes the potions master quite well.
Of course, it doesn't just stop with the names either, there are places and creatures that all have mythological ties...Hippogriffs, Centaurs, Boggarts...etc