Open Content on JSTOR, a new single-page listing of links to all public-and-open content at JSTOR. 2,260 OA books, monographs and reports. 339 pre-1923 journals. You can also do a title search.
All the early pre-1923 journal content should be found in JURN, via selective indexing which targets the pre-1923 JSTOR mirror on Archive.org. The bulk (possibly all) of the OA books are also found in JURN, via picking up record pages at one of the book publisher catalogues (e.g. RAND Corp., which supplies nearly 1,000 of the JSTOR books) and/or on OA book catalogues such as oapen.org. JURN doesn’t index DOAB, as it’s not needed — its content is all covered elsewhere.