BtVS by Whedon and Mutant Enemy. The (more than) trilogy-cubed novels of Xanth are by Piers Anthony. Takes place some time past 'Currant Events' and during the summer after 'the Gift'
Challenge #959 had me pecking keys with my eyes closed and assigning meaning to the result, four letters for subjects and a number for word count. Mine was GDIO-5(00). 'G'iles was easy and a crossover meeting with 'D'or sounded interesting. 'I'nitiative came next as I'd decided to write about dragons. Stumped, the next 'o' to pop up in my botany class: 'O'rganisms.
What happens during Dor's conversation is an example of his natural-born ability as, to be a King in Xanth, you need a Magician caliber Talent. Incidentally, it's the official title - when women ascend to the highest rung of power in the human government they're still 'Kings', generally to the annoyance of their spouses. Also, while are Talents are unique in definition the results can overlap – the kid in the opening bit is less powerful than 'Hiatus'.
Giles was sitting in the waiting room of the palace, drinking some bottled water, when his own backpack opened an eye and winked at him. Hearing teenage laughter in the background, Giles resolutely ignored the apparition until both it and the boy who was causing it went away.
“King Dor,” Giles began, once he'd been allowed to enter the throne room and bow before the man. “I have come from a different, magically-active version of Earth than the one you visited, in order to ask for help. Not in the form of magical weapons or powerful magicians, as I’m to understand they wouldn’t work reliably, but in the form of skilled personnel.
“You see,” Giles continued. “We had a single dragon fly through a multidimensional breach and take up residence in our reality. There’s ten of them at the moment and we haven’t had much luck in reducing their numbers. Our military, or at least a segment of it called the Initiative, has been called in to do battle with the creatures, with very mixed results. This metaplane lacks the exact species we’re facing, but I believe your warriors can ‘think on their feet’ and know enough dragon fighting theory
to improve our chances of containing the ‘problem’. We, I, uh, lost my best... friend in repairing the breach in the first place so we’re very...”
“I understand,” the king said, picking up the thread of the conversation. “We recently imported several thousand dragons from a compartment reality in order to stabilize our rapidly declining ecosystem. As humans are definitely part of the local ecology we’ve also been training a fresh crop of dragonslayers, many of whom are dedicated fighters with only Spot-on-the-Wall Talents who would be honored to train and potentially shine under such harsh conditions,” Dor paused mid-sentence, noticing that Giles had removed his glasses in order to rub the bridge of his nose. “Well, what do you have to say about it?”
“You see,” the pair of glasses said, causing Giles to nearly drop them in shock. “Hey, careful there!”
“Sorry,” Giles apologized, holding them at arm’s length and steadying his grip. “I was briefed that something like this might happen, but I wasn’t expecting the accent.
“S’okay,” the glasses cheerfully announced. “Everything my man said here is true, but we don’t trust those Initative wankers, in principle. He’s also fairly pissed off at his old home team, the Watcher’s Council, who ‘arranged’ the release of an unstable Slayer named Faith from jail, yet haven’t let a single text cross the ocean.”
“Yes, well, the bastards had fired me unfairly so I truthfully wasn’t expecting much help. What makes it worse is that they’re parthenogenetic and oviparous organisms, meaning like aphids they’re born pregnant,” Giles said, unconsciously cleaning his glasses until they started purring. “Basically they’re multiplying like rabbits.”
“Yes,” Dor nodded sagely. “Rabbits are some of our best mathematicians, however they don’t get a lot done because they’re always splitting hares.”