What are some good books out there regarding gene therapy? I'm interested in introductory literature.

While not exactly a "book", have you seen the May 2004 issue of Biopharm magazine? It carried an article on the science behind the world's first licensed gene therapy drug being manufactured by SiBiono -- one of China's leading gene therapy companies. The same issue also published the first English translation of guidelines for production of human gene therapy drugs -- which the Chinese govenment have now adopted as their official standard. If you can not obtain the publication readily, there is a PDF of the article on the website address shown below: http://www.nbsc.com/products/bioreactors/CelliGenPlus.asp

One of the best introductory books is :
Understanding Gene therapy


It's not too expensive. It starts at the begining with DNA, then goes on to consider different vectors and applications.

It's well referenced so it's a good starting point for further reading.

Here is a great web-site regarding "Intro to Gene Therapy" - I'm sure you could follow up to obtain literature from cited sources. See link below:
http://laxmi.nuc.ucla.edu:8237/M288/lec/chow-detail.html