There is an excellent ski touring book of Lassen Volcanic National Park called, oddly enough, “Ski Tours in Lassen Volcanic National Park” by Marcus Libkind (Mountain Sports in Chico at 176th E. 3rd Street, Downtown Chico carries it).
The northern access to Lassen Park at Manzanita Lake is more gentle than the Southwest entrance, offering some nice meadows to cruise through on both sides of the road. The two main trails to use are the Old Emigrant Trail and the trail up to Crescent Cliffs. Ask the ranger at the visitor’s center how to find the trailheads.
During the Spring months, the trans-park road is first plowed up to the Devastated Area, then full attention is given to removing snow from the southern end.
When the Devastated Area is accessible, there are some nice options for skiing. Just a little ways up the road is the start of the Nobles Trail, on which Emigrants once brought their wagon trains. Further up the park road is Summit Lake, one of the biggest summertime campgrounds in the park. Plenty of ski opportunities abound.