Books and journal articles discussing or analyzing Darwin's notes, sketches or theories in depth, are SECONDARY sources. Secondary sources, which interpret, analyze, or otherwise filter primary, or other secondary, sources, are explained here.
Use Google Scholar to find academic-quality information (articles, papers, reports) on the Web.
Frequently, a source that is not a primary source is a SECONDARY source. Typically, secondary sources comment upon, analyze, or draw information from primary sources. Secondary sources can also interpret, critique, or explain primary sources.
Like this e-book about Charles Darwin and his science, a secondary source analyzes, interprets, filters, or otherwise discusses primary source material.