Some of the rest of Diana Wynne Jones' stuff liiike:
dark Lord of Derkholm (there are several main couples and I'm pretty sure a couple have a love hate dynamic...)
Tamora Pierce is awesome. She's written a lot, and they can be read in order or just read one of the quartets at a time or whatever:
The Tortall series:
- Song of the Lioness Quartet (main couple doesn't initially hate each other, but the beta couple does)
- Immortals Quartet (works on all counts)
- Protector of the Small quartet (doesn't work for #2)
- Trickster's Choice and Trickster's Queen (works on all counts)
- Provost's Dog Trilogy (works sooo well for #2. And they're adorable. And awesome)
The Circle of Magic Series (composed of two quartets and a few other books, but they don't have so much going on in the romance department...)
Gemma Doyle Trilogy by Libba Bray (splits it's time half way between the "real" world (in Victorian England) and a magical world. Has a couple who hate each other. Magic is used)
Tithe by Holly Black (it's "urban" fantasy, so it's set in modern day... not quite like Howl's. But they do hate each other for awhile)
The Talking to Dragons series by Patricia C Wrede is good as well (takes place in a magical world, magic is used, couple fights alot... though the love interest isn't introduced until book #2... and #4 has a different main couple... but they hate each other too) but it's somewhere between "kids" and "YA".
Another "younger" (even then the previous example) would be the Sisters Grimm series, though I myself have not read it, I've heard it's good...
Neil Gaimon's book Stardust is a perfect example of this.