If you are interested in material in the novels and not just the sourcebooks, have a look at The Ruin, the last book of The Year of Rogue Dragons trilogy by by Richard Lee Byers. Some of the events take place around Sossal and involve the Sossrim.
AD&D 2e sourcebook Warriors and Priests of the Realms provides information (pages 41-44) in the section "Warriors of the Cold Lands", where 4-5 paragraphs of fluff is present along with a kit for playing Sossrim warriors. The kit has a rather unusual disadvantage: once the character reaches 7th level, she/he must kill a white dragon as a rite of passage or else they cannot advance further.
For other sources, make sure you also check out the following Candlekeep discussion, which contains an extended list of Sossal-related material: http://forum.candlekeep.com/topic.asp?TOPIC_ID=4445