A boy and girl go on adventures. One of them involves a merman who becomes a misunderstood monster, maybe, but they turn him back to normal. On this adventure they meet a squid which gives them ink and a blackbird/crow drops the ink on a mean girl's head.
Another adventure involves defeating an evil snow/ice queen who later on, probably in another book becomes an evil sun empress. The girl is from a normal world while the boy is from the fantasy world and works for someone.
Not sure when it was published but it's easy to read with no ye old English. Probably for 10 year olds, I read it about 10 years ago.