Bielefeld Academic Search Engine (BASE) searches metadata and some full text, from over 2000 sources of academic open access documents. About 60% of the documents indexed are available full text. The metadata searched is provided by the source and tagging is often inexact. This is a vast collection of documents and has much not available elsewhere.

Use Browsing to narrow your search to subject area (e.g., Literature) or Document type (e.g., Manuscript, broadly construed). Open Browsing and choose-Dewey Decimal (for Subject), choose major subject to see next finer level, twice. After choosing View Records add a search term to the Subject Term or Document Type.

Link: https://www.base-search.net/