National Trust for Historic Preservation’s easy-to-understand illustrated steps for those making first attempts at researching their home’s history. Their ‘Preservation Tips and Tools’ section is designed to help anyone save historic places in their own community. The recommendations are practical and the resources are easily accessible. A good introduction.

Published by the National Trust for Historic Preservation, a private, non-profit organization; the authors are listed as Emily Potter and Priya Chhaya, but no other information is provided.
Online article.

An online archive of architectural trade catalogues, house plan books and technical building guides printed before 1964. To date, the BTHL has more than 9,800 documents on its site. This could be helpful for someone looking for original specs and technical information, capitalizing on the fact that because modernism is so recent, many of the original plans and specs are still available, or someone looking for popular designs of the time for ideas and inspiration.

Published and maintained by Internet Archive in partnership with the International Association of Preservation Technology (APT).
Online archive of images and complete original documents, downloadable in a few formats.

This is an open digital collection of major US 20th-century architecture magazines with approximately 2.1 million downloadable pages, all free. Great for researching and viewing original source material and articles.

Managed and published by usmodernist.org, a non-profit educational archive.
Online archive of 20th century architecture magazines, downloadable.

Vast network of homeowners, vendors and services focusing on, though not exclusive to, the Northern California mid-century developments of Joseph Eichler. Back issues of their newsletter (1996-2005), their current publication CA-Modern are for sale on the site, as well as numerous articles covering maintenance issues, preservation, and an updated blog.

Published by the Eichler Network.
Website and blog format.