Rocannon's World, by Ursula K. Le Guin
This one must be one of the earliest Ekumen novels - internally and externally. It was written in 1966, and it covers a time before "the fall" when the League of Worlds is seeking allies against a mysterious unspecified enemy who is coming from another galaxy. It reads a bit like a fairy tale, despite clearly being science fiction, by taking on a bit of the feel of the feudal society it describes.
A young wife to an impoverished lord sets out to regain a famous treasure lost to her family generations before. She speaks to some flighty elf-like creatures who send her to see some troglodytic dwarves, who in turn take her on a journey in strange mechanical conveyances to a strange museum. There she asks a man for the jewel, he gives it to her, and she returns home to find decades have passed, her husband is dead and her infant daughter grown.
It turns out, of course, that the strange conveyance was a near-lightspeed starship which the dwarves were given by the League, in an attempt to build them up technically to the point where they could help in the war; in a ritual exchange of gifts the dwarves had given the jewel, but the League didn't really care about it so they gave it back when asked. The strange museum was on another world, and the relativistic travel speeds are what caused the missing time.
Rocannon is the man who saw the girl and gave her the jewel, and intrigued by the little known about her race - and somewhat haunted by her beauty - he organises an expedition to the planet to re-evaluate the way the League is dealing with only the dwarves. While on the planet a rebellion occurs in the League, and the rebels destroy Rocannon's ship, his teammates, and his only method of communication with the rest of the galaxy. He sets out on a journey with his companion - the grandson of the woman he was haunted by - to reach the enemy base and somehow warn the rest of the League. The journey is long, arduous, quite interesting, and ultimately difficult to satisfyingly summarise, so I won't. Go read the book. In the end he learns what amounts to telepathy from a strange being in the mountains, but has to pay a high price for his newly won skills. He uses this new ability to sneak onto the enemy base and accomplish his goal, and is presumably offstage the manner in which the human race learns telepathy.
As usual for Ursula, a well-told story which follows some interesting consequences of technical ideas (relativistic travel but FTL communication, etc.) but is ultimately more about people, and how they cope with the situations they find themselves in. This is the first time I'd read this one, and it was interesting to get that much more of the back story to what comes after.