Journal articles (particularly in science, health and social science) have proved the easiest to make available via open access, as many journals are now produced electronically rather than in print. As a result, many research funders with policies on open access have focussed on journal articles, and also conference proceedings.
As yet books and book chapters have not been a focus for making content open access. However, there are moves to change this.
Some publishers allow individual book chapters to be made open access from their websites, or for the chapters to be made accessible through repositories after an embargo period (often longer than journal embargo periods), and this option is well worth exploring.
The cost of monograph publishing has led to several projects and initiatives to investigate what is possible around open access to books. There are many different models for open access monograph publishing that are currently being explored, and while some of these options are quite expensive, some less traditional models for covering publication costs are being trialled, so if you are interested in making your book open access there may be a suitable option available to you. Note that researchers receiving Wellcome Trust funding are required to make any books resulting from their research open access (and Wellcome trust will cover the costs of this).
Many universities worldwide offer the opportunity for doctoral students to submit an electronic copy of their thesis, which can be accessed much more widely than the traditional print copy.