From the Delft Technical University, this list of Open Educational Resources is a great starting point for finding Open Access Educational Resources. After opening the pdf you can go to the individual websites via the icons with each item.
A NUMBER OF OPEN TEXTBOOK INITIATIVES
A NUMBER OF OPEN E-BOOK INITIATIVES
The National Academies Press (USA) : Sciences, Engineering and Medicine
InTech Open : multidisciplinary scientific literature
The World Bank Open Knowledge Repository: research and knowledge products on development.
UN-Habitat: water and sanitation, the environment, urban management, land and housing, slums, women’s rights.
Food and Agricultural Organization of the UN: all FAO publications full text online
Directory of Open Access Books