Threshold concepts are by definition:
Transformative - these concepts once learned by the learner open up new ways of understanding the world and questioning of former ways of thinking.
Integrative- learners are able to integrate threshold concepts into other contexts and related areas.
Irreversible - threshold concepts once learned are difficult to unlearn.
Bounded - threshold concepts are contextual they are learned within a particular context and often along with and in addition to other threshold concepts.
Troublesome - threshold concepts can be potentially be troublesome to the learner.
Source: Blackmore, M. (2010) Student engagement with information: Applying a threshold concept approach to information literacy development. Paper
First of all you might want to check out the official ACRL site on the Framework: http://acrl.ala.org/framework/
There is also an extensive bibliography put together by the ACRL Student Learning and Information Literacy Committee: http://www.ala.org/acrl/issues/infolit/teaching/thresholdbib
This is our short list of recommended readings about threshold concepts:
Based on participant feedback from the March 11, 2015, webinar on the ACRL Framework for Information Literacy for Higher Education, has created a listserv to provide a convenient spot for exchanging ideas about the Framework.
The new list will serve as an open forum for asking questions, posting professional development opportunities, and sharing examples of how you are using the Framework at your libraries. To subscribe, visit http://lists.ala.org/sympa/