The present invention concerns isolation, library preparation and selective amplification from a compositionally heterogeneous pool of DNA fragments of a fraction of molecules, such as those originating from promoter CpG islands and characterized by a high GC content. In particular, the process utilizes a heat-induced segregation of DNA molecules into GC-poor, single-stranded molecule fractions and GC-rich, double-stranded molecule fractions, with subsequent enzymatic conversion of the GC-rich, double-stranded DNA molecules into a library, and, optionally, amplification. In specific embodiments, the isolation process is used to generate promoter-enriched genomic and methylome libraries for research and diagnostic applications, for example.