Encountered: Chapter Seven - in the library, Chapter Eight - in the library and mentioned later in Seth's home, Chapter Ten - in the streets of Huntsdale.

The Summer Girls are earlier girls, chosen by Keenan, sometime during the past nine centuries as 'the one'. The Summer Girls are the girls who didn't choose to lift the staff and try their luck at becoming Summer Queen; there is at least eighty of them. They need to be around Keenan to thrive--otherwise they'll fade. It is unknown how long they can stay away from him, or if the events of Wicked Lovely have changed how close they need to remain to him.

'The door opened and a mother with a couple of kids came in.
Behind them was a group of faeries, invisible to the other patrons. All six were beautiful - moving like models, wearing clothes that looked like they'd been tailored for their willowy bodies. If it weren't for the flowering vines slithering across their skin, they'd look human. The vines, though, were like living tattoos, moving of their own volition, crawling on the girls' bodies.
One of the girls span across the floor in some old-fashioned dance. The others giggled and bowed to one another before following her.
Then the first one saw Donia. She murmured something to the others and they stopped. Even the undulating vines stilled.
[...]
Finally, Aislinn said, "If you hadn't been there..."
"What?" The expression on Donia's face was pained as she looked away from the faeries.
[...]
Donia shot a brief glare at the faeries, who were giggling again.' (pg77 and pg78)

In Chapter Eight, after Donia leaves and Seth arrives, he leaves her side briefly to grab notes, and the Summer Girls come closer to Aislinn:

'"Let me grab my notes." Then he walked away, right past the group of faeries headed towards Aislinn.
One of the faery girls circled behind her. She's the new one.
A second one stroked a hand over Aislinn's hair. Pretty thing.
Another shrugged. I suppose.
Aislinn tried to keep her face blank. Focus. She concentrated on the rustling of the leaves against the girls' clothes, not the strange sugary-sweet scent that seemed to pervade the air around them, not the too-hot brush of their skin as they inspected her with their hands. It wasn't comfortable - at all - but after the fiasco outside, their touch seemed somehow less awful. The violence of the three guys... She shuddered.
The faeries chattered back and forth, louder now that Donia had left and, presumably, no one in the library could hear them.
The Winter Girl seems to be making progress.
This one's a no-touch now.
Who cares? I'm not fond of girls. Now her friend... He's touchable. Tasty.
They giggled.
If she's the one, she won't have a choice, will she? Her friend will be free game.
[...]
Who says we need to wait? One of the faeries stroked his cheek, another pinched him.
Seth's eyes widened.
Aislinn's heart thumped. He felt it. She'd never had to try to speak so the faeries didn't understand her, not with anyone but Grams, not with anyone who couldn't see them. Hoping the faeries were as daft as they looked, she slid her arm around his waist and tugged him towards the door, away from the lascivious faeries. "Ready to go home?"
"Definitely." He sped up a little, and draped an arm over her shoulders.
The Summer King might have some competition.
You want to tell him that? "Oh, Keenan, love... her toy is yummy."
Don't be mean. The king's good fun.
They all giggled again.
How much fun will he be with her around? You know how he gets.
I'll volunteer to distract the mortal, so Keenan can woo her.
Mmm, me too. Look at all those rings on his face. Wonder if he's got a tongue ring?' (pg80-pg82)

Aislinn mentions the Summer Girls again a few pages later:

'Finally she pulled back a little and told Seth about the faeries at the library who'd been circling her and talking about him. Then she asked, "So what do you think?"
He wrapped a long strand of her hair around his finger and stared at her. "About tongue rings?"
"About the faeries' comments," she corrected, blushing. She slid forwards like she was going to hop off the counter. "They seem to know what's going on, maybe you could see if there's anything about groups of Rianne-like faeries? You know, one's that are overly shallow and, umm, Seth..."' (pg85)

They are encountered again briefly (it is unknown if the Girls are the same as those earlier encountered) in Chapter Ten:

'Seth held on to her, but he didn't say anything.
"What would I do without you?" She closed her eyes, not wanting to see the vine-girls - or any of the other faeries - who stood watching them.' (pg104)