Book with people called the Sidhe [duplicate]

Question

A while back, I read a fantasy book about a boy who ends up in another world inhabited by a people called the Sidhe (I think). He got there while visiting an old man at night, but something spooked him and he ran into the garden, where he went through a portal. He was taken in by three witches, until he went his own way. I also seem to remember he could either talk to horses or transform into one. I think the main storyline was that the Sidhe world and the ordinary world were getting too close to each other. Its all very vague.

It was not written by Feist, Asimov, Prachett, Jordan, Tolkien. I read it back in the early 2000's, but I don't know if that was when it was published. I believe it was part of a series, as I seem to remember part two took place in the "normal" world. I also seem to remember the protagonist (boy) was either part Sidhe/part human, or had at least a connection to them.

Answer 1

It could be Songs of Earth and Power by Greg Bear. It was originally published as two separate books, The Infinity Concerto and The Serpent Mage.

The opening is what I'm basing my guess on, where you mention he visits an old man's home and ends up in a garden, then the realm of the Sidhe. How it goes in the novel is the protagonist, one Michael Perrin, had an old composer friend who recently passed away. He is passed a list of specific instructions that will take him through the old mans home, out through the yard and then through the house next door (reality is already shifting on him as this goes), then through the hedge garden that he finds himself in.