Free, online encyclopedias like the Stanford Encyclopedia of Philosophy and the Internet Encyclopedia of Philosophy contain entries on all sorts of topics in philosophy, broad and narrow -- all of which are written by professional philosophers. Peruse these encyclopedias to enhance your background knowledge of topics and major figures in philosophy as well as your understanding of intricate concepts you're learning about in your philosophy courses.
From its inception, the SEP was designed so that each entry is maintained and kept up-to-date by an expert or group of experts in the field. All entries and substantive updates are refereed by the members of a distinguished Editorial Board before they are made public. Consequently, our dynamic reference work maintains academic standards while evolving and adapting in response to new research.
From "About the Stanford Encyclopedia of Philosophy." Read more about the SEP.
The purpose of the IEP is to provide detailed, scholarly, peer-reviewed information on key topics and philosophers in all areas of academic philosophy. The Encyclopedia's articles are written with the intention that most of the article can be understood by advanced undergraduates majoring in philosophy and by other scholars who are not working in the field covered by that article. The IEP articles are written by experts but not for experts in analogy to the way the Scientific American magazine is written by scientific experts but not primarily for scientific experts.
From Internet Encyclopedia of Philosophy's "Statement of Purpose." Read more about the IEP.