https://www.unddr.org/

Basically the center of this, the UN truly lies at the center of DDR and SSR efforts. It's important to note that the field has largely begun to shift away from DDR and SSR as a whole and more towards preventing violence and outbreak of conflict.

https://peacekeeping.un.org/en/disarmament-demobilization-and-reintegration -

Chances are, if you're reading this post, you've already read this before. But if not, this provides some good up-to-date information about where and how DDR and SSR are being carried out.

https://peacekeeping.un.org/sites/default/files/the_changing_landscape_of_armed_groups.pdf -

This report is a good introduction to what is called "next generation" DDR. DDR has gone through three iterations: the first in the 90's in response to peace accords and formal military structures in Africa, the second a "integrated" DDR iteration that incorporates the well-being of minority groups and focuses on a more flexible approach, and a "next generation" DDR iteration that incorporates armed non-state actors.

http://gsdrc.org/wp-content/uploads/2016/06/HDQ1368.pdf

This short report gives a broad-based overview into the various DDR iterations.

https://peacekeeping.un.org/sites/default/files/un_ddr_in_an_era_of_violent_extremism.pdf

This addresses some of the shifts in the field lately.

http://www.iddrtg.org/resources/

I'm a little bearish on the usefulness of this website, but this group collaborates with the design and implementation of DDR groups.

• https://reliefweb.int/report/angola/multi-country-demobilization-and-reintegration-program-end-program-evaluation-final
• https://reliefweb.int/organization/tdrp

The Multi-Country-Demobilization Program (MDRP) and the Transitional Demobilization and Reintegration Program (TDRP) are two of the largest multi-country DDR programs in the past two decades.

Muggah, Robert, and Chris O’Donnell. “Next Generation Disarmament, Demobilization and Reintegration.” Stability: International Journal of Security & Development 4, no. 1 (May 21, 2015). https://doi.org/10.5334/sta.fs.

This paper establishes what we could conceivably call "next generation" DDR.

https://www.e-elgar.com/shop/usd/research-handbook-on-post-conflict-state-building-9781788971638.html

This book is actually quite broad and zooms further out than just DDR and SSR. While it's largely not that good at giving practitioner level advice, but does give a useful organizational framing for post-conflict state-building. I found this quite useful for framing my understanding of DDR.

https://papers.ssrn.com/sol3/papers.cfm?abstract_id=1906329

This is a nice and short literature review from 2010 about DDR.