I always thought that Boba Fett was natural offspring of Jango Fett. But, I just learned that Boba Fett was clone of Jango Fett (Jango Fett only raised him as child). Is this in the new canon established by Disney? If not, which book from Legends establishes this?
Yes. It is canon.
LAMA SU : Apart from his pay, which is considerable, Fett demanded only one thing - an unaltered clone for himself. Curious isn't it?
OBI-WAN : Unaltered?
LAMA SU : Pure genetic replication. No tampering with the structure to make it more docile... and no growth acceleration...
OBI-WAN : I would like to meet this Jango Fett.
INT. TIPOCA CITY, CORRIDOR - DAY
TAUN WE conducts OBI-WAN to the door of Jango Fett's apartment. TAUN WE waves his hand, and a muted bell RINGS.
As they wait, OBI-WAN notes the door lock entry mechanism. Then the door opens, and a ten-year-old boy, BOBA FETT, looks at them. He is identical to the boys in the classroom.
TAUN WE : Boba, is your father here?
There is a brief pause, then BOBA FETT nods.
TAUN WE (continuing) : May we see him?
BOBA FETT : Sure.
Now, just going off the movie/script, one could make the argument that they never explicitly specify out loud that Boba was the unaltered clone (they say that Jango asked for an unaltered clone, and then we meet Boba living with Jango), but StarWars.com specifically states this to be fact.
With his customized Mandalorian armor, deadly weaponry, and silent demeanor, Boba Fett was one of the most feared bounty hunters in the galaxy. A genetic clone of his “father,” bounty hunter Jango Fett, Boba learned combat and martial skills from a young age. Over the course of his career, which included contracts for the Empire and the criminal underworld, he became a legend.
It is a fully canon fact. In Episode II, Lama Su tells Obi-Wan:
Apart from his pay, which is considerable, Fett demanded only one thing: an unaltered clone for himself. Curious, isn't it?
The clone is obviously Boba.