In this talk, I will present recent results on a new sample of extremely UV-luminous star-forming galaxies at z=2-4 discovered within the 9000deg^2-wide Baryon Oscillation Spectroscopic Survey database of the Sloan Digital Sky Survey. These puzzling sources show apparent magnitudes rivaling those of bright QSOs, but without any hint of AGN activity or being magnified by gravitational lensing. Instead, these sources are characterized by very young stellar populations (~ 10 Myr) and compact morphologies. The two highest-redshift sources in our sample show very high Lyman continuum (LyC, with >13.6 eV) escape fractions, up to fesc(LyC)~90%, being the most powerful ionizing sources identified so far among the star-forming galaxy population, both in terms of the intrinsic LyC photon production rate and escape. With SFRs~1000 Msun/yr, but almost un-obscured, and specific star formation sSFR >50-100 Gyr^-1, these sources are very efficient star-forming galaxies, possibly representing a short-lived phase in the evolution of massive and compact galaxies. I will highlight some unique properties observed in these sources including LyC emission, complex Lyman-alpha profiles, strong wind lines, SEDs, among others. Finally, I discuss the properties of these UV-bright sources in the broad context of galaxy formation and evolution, and possible implications to cosmic reionization.