Background or "reference" sources are a great place to start in building your bibliographies. These are scholarly encyclopedias, handbooks, bibliographies, and similar sources that will often contain lengthy essays with background information on a topic and overviews of the relevant scholarship. Citations to the secondary literature will always be included, and often so too will citations to primary sources.
Wiley Online Library
Relevant titles such as A Companion to American Environmental History will provide overviews of topics and suggestions for further reading.
Oxford Research Encyclopedia in American History
The articles here are arranged into subfields and include "American Environmental Policy since 1964," "The American Antinuclear Movement," and more.
The Oxford Handbook of Energy Politics
This and other Oxford Handbooks are useful sources for getting started on research projects. You will find informative overviews on topics and helpful lists of suggestions for further reading in each chapter.
Oxford Bibliographies
This is a large collection of bibliographic “articles,” each containing numerous bibliographic annotations, often of both primary and secondary sources.
Gale Virtual Reference Library
Includes the International Encyclopedia of the Social Sciences and other sources useful for quick references.