First as a sort of disclaimer, I am still learning Japanese, so while I feel I understood the story, there may be some things that I missed. I would love to hear your perspective if you've read it. =)
A girl named Ningyo (yes - 人魚) has lived with her mom for all her life. Her dad lives somewhere else, with another woman and her son. Ningyo's mom is sure she was cheated on (making the boy Ningyo's half-brother), but her dad insists that the boy isn't his.
Then I think the lady dies, and Ningyo’s parents not only get back together but end up in Nepal, leaving Ningyo alone for the first time in her life. She is pretty lonely, but luckily meets Arashi, with whom she immediately feels an affinity - - and of course it is the guy who might be her half-brother.
The rest of the story kind of goes from there – they chat about their lives and childhoods and parents, they talk with the 2 in Nepal, Ningyo wonders if/when her mother will return, she seeks counsel from her dad, wonders if Arashi is her brother, etc. There are a few exciting/ notable events, and the ending is pretty in line with what I know of Yoshimoto.
I have always heard that her work is easy to read, and I'm finally getting to the level of Japanese where I can agree! The grammar was fairly simple, and the dialog didn't stray too much from standard dialect, so I feel that even if you mostly know "classroom Japanese", you'll be able to follow it. While my vocabulary is still on the smaller side, I didn't feel compelled to look up very many words, since I was able to infer meaning from the rest of the sentence. The whole thing clocks in at 100 pages, and is pretty light (no dramatic twists and turns), so for me it was a pretty good first book.


Interestingly, in the afterword at the back of the book the author says something to the effect that looking back she doesn't think these two stories were really written in her own style, so she's not sure she likes them... (I'm paraphrasing from memory as I don't have the book to hand.)
If you like Yoshimoto Banana I think my favourite of those I've read so far is キッチン: it's a bit longer than うたかた but not otherwise any more difficult a read.
嵐とは一回キスしただけだ。
ここが日本だからまだよかったが、外国だったらそんなのほとんど友達以前の範疇だ。そ
それでも嵐を好きになってから私は、恋というものを桜や花火のようだと思わなくなった