What unforeseen dangers might exist when choosing an open access textbook for a new subject?
It's important to check the license used by the author to make sure that it's compatible with your use of the open access textbook.  

Issues to check include:


Does the license allows you to redistribute the book to your students (e.g. by putting it on your course web site) or whether students have to get it from the author's web site.  I would not be willing to use any material that I couldn't distribute to my students, because the author might pull it from the web at any time.  
Does the license allow you to modify the work (e.g. fix typos or more broadly edit the work)?  How are you required to describe any modifications?  
If there are any restrictions on "commercial use", does your course constitute commercial use?  Some people have argued that for-profit higher educational institutions can't use Creative Commons NC (CC-NC) licensed materials in courses.  


Many open access educational resources are licensed under the Creative Commons license with varying options (CC-BY, CC-NC, CC-ND, etc.)  The Creative Commons web site has clear explanations of how those licenses work.  Many other resources have been put up on the web with no specified license.  If there's no license specified you should contact the author and ask for permission to use the material.