Check out their page http://www.gao.gov

You can find a list of some of their available material at
Special Publications: Evaluation Research and Methodology
http://www.gao.gov/special.pubs/erm.html

Program Evaluation: A Variety of Rigorous Methods Can Help Identify Effective Interventions
GAO-10-30 November 23, 2009   http://www.gao.gov/Products/GAO-10-30   Several rigorous alternatives to randomized experiments are considered appropriate for other situations: quasi-experimental comparison group studies, statistical analyses of observational data, and--in some circumstances--in-depth case studies. The credibility of their estimates of program effects relies on how well the studies' designs rule out competing causal explanations. Collecting additional data and targeting comparisons can help rule out other explanations. GAO concludes that (1) requiring evidence from randomized studies as sole proof of effectiveness will likely exclude many potentially effective and worthwhile practices

These others are all old, but still very good.

Designing Evaluations. PEMD-10.1.4. May 1991. Guidance. 78 pp.  http://www.gao.gov/products/PEMD-10.1.4   or pdf version here  http://archive.gao.gov/t2pbat7/144040.pdf    and here  http://www.gao.gov/special.pubs/pe1014.pdf    

Case Study Evaluations PEMD-10.1.9, November 1990   summary here   http://www.gao.gov/products/PEMD-10.1.9  and full report is here   http://www.gao.gov/special.pubs/pe1019.pdf   

The Evaluation Synthesis PEMD-10.1.2, March 1992   summary here   http://www.gao.gov/products/PEMD-10.1.2  full report here   http://www.gao.gov/special.pubs/pe1012.pdf 

Performance Measurement and Evaluation: Definitions and Relationships. GAO-05-739SP, May 2005.   http://www.gao.gov/new.items/d05739sp.pdf      

Quantitative Data Analysis: An Introduction. PEMD-10.1.11. June 1992. Guidance. 110 pp   summary here   http://www.gao.gov/products/PEMD-10.1.11   full report here     http://www.gao.gov/special.pubs/pe10111.pdf 

Using Structured Interviewing Techniques. PEMD-10.1.5. July 1991. Guidance. 92 pp.   http://www.gao.gov/special.pubs/pe1015.pdf 
    abstract here   http://www.gao.gov/products/PEMD-10.1.4  

A couple of other books at the GAO website not listed there are these.

Developing and Using Questionnaires PEMD-10.1.7 October 1993 238 pp.   summary here   http://www.gao.gov/products/PEMD-10.1.7   full report here   http://archive.gao.gov/t2pbat4/150366.pdf   

Using Statistical Sampling, PEMD-10.1.6  summary here   http://www.gao.gov/products/PEMD-10.1.6   full report here   http://archive.gao.gov/t2pbat6/146859.pdf    

Content Analysis: A Methodology for Structuring and Analyzing Written Material. PEMD-10.3.1. September 1, 1996. Guidance. 57 pp.   http://archive.gao.gov/f0102/157490.pdf   
abstract here  http://www.gao.gov/products/PEMD-10.3.1