Really ? It seem to indicate that it is only a "historical novel". If that is the case, then it should be seen as the likes of "The Other Boleyn Girl".
The cover of Pilar Eyre's book clearly states that it is a novel.
As far as ascertaining the truth or otherwise of this suggestion, the starting point must be whether Elizabeth and Alfonso actually met in 1913.
If they did not, the story is likely to be baseless.
According to Ana de Sagrera's book "Ena y Bee", which is a reliable source, Elisabeta did visit the Spanish court and met Alfonso XIII. The book states that he was charmed by her, but it doesn't state that they had an affair.
Pilar Eyre, author of the new book on Queen Sofia of Spain, writes in El Mundo (in an article on Spain's social life in 1913) that:
"She tells a party at the palace of the Marchioness of Esquilache in 1913. The King and Queen attended, as well as Beatrice of Coburg (Infanta Beatriz). The party was to in honour of Princess Elisabetha (Princess Elizabeth of Romania). After dinner and a ball with the Queen, the King did not stop dancing with Elizabeth. According to the article, the Princess had come to Spain with clear intentions: "I have tried all possible vices. Let's see if they teach me something new here!". Next day, the King took the Princess in his car for a visit and, in the evening, another party at the palace of the Marchess of Casa Valencia. Next morning, the King went to the Orleans palace to take Elizabeth for some ice skating. In the evening another ball for the inauguration of the palace of the Prince and Princess of Pless. A lavish dinner and fireworks. The King and the Princess got "lost" in the gardens. A paper next day made reference to a Princess and a high personality of the Kingdom. Due to the gossip, the Princess had to leave Spain."
courtesy Manuel Franco-Iberian Message Board