The Norwegian series Phenomena has only been released in Norway, Sweden, Denmark, Finland and Germany. It sells well in both Norway and Germany, but still no word of it ever to be given out in other countries.
Another Norwegian series Halvgudene has only been translated into Icelandic.
Despite excellent sales for Discworld novels in the United States, the spin-off series The Science Of Discworld is unavailable from most American booksellers.
Russian novel Loyal Enemies has been translated only to Polish so far and is available only in Eastern Europe.
While Harry Turtledove's Worldwar books were translated and released in the Russian-speaking countries. The Colonization series and Homeward Bound were not. The culprit here is not Turtledove or his publisher but the Russian publisher Eksmo, who bought the rights to the book and then sat on them (and are still sitting), refusing to translate and release the novels for "ideological reasons". They claim that they don't like how the USSR is portrayed in the novels, despite the fact that it was portrayed much worse in Worldwar (in fact, Russians are far from being central to the main plots of Colonization or Homeward Bound). Additionally, the existence of Nazi Germany in the 60s was claimed as "distasteful", despite the fact that there are plenty of Alternate History Russian novels that do the same.
A strange case: the final two (of five) novels in the Kantmorie series by American author Ansen Dibell (Tidestorm Limit and The Sun of Return) have only been published in Europe, and are only available in French and Dutch translations.
Return To Rainspell Island has yet to come out outside the UK.
A few series haven't made it to the US, such as the Talent Show Fairies, and a few books about fairies that protect the royal family..
While Rosemary Wells's books have been translated in English, French and Spanish. Some of her books such as the more recent entries to the Max and Ruby series, Noisy Nora, Shy Charles, and the Yoko series aren't translated in other countries.
Two of Mitsuyo Kakuta's novels have been translated into English. If you enjoyed those two and want to read more, then tough luck—she's written lots of other books, but only two have been translated.
Vertical Books began a project to translate Guin Saga into English, with the first three volumes released in hardcover in quick succession in 2003. Four and a half years later, those same three books were released in paperback, along with the fourth and fifth books. Thus ended English translations of Guin Saga...a series of more than 130 books.
Very little Byzantine literature has ever been translated into English. Admit it, you didn't even know there was any, did you?
Sitio Do Picapau Amarelo by Monteiro Lobato is considered the Brazilian equivalent to Chronicles of Narnia and has been translated to both Russian and Spanish languages... But never officially translated to English. Its especially mind-bogging given the author was personally responsible for translating Tarzan and the Jungle Book to Portuguese language.