Yocto Project documentation: there are nine reference guides plus a tenth which is a composite of the others (the so-called Mega-manual") at https://www.yoctoproject.org/documentation
Instant Buildroot, by Daniel Manchón Vizuete, Packt Publishing, 2013
Embedded Linux Development with Yocto Project, by Otavio Salvador and Daianne Angolini, Packt Publishing, 2014