I’ll add two other references to Ben’s great response:
1. Effective Work Breakdown Structures by Haugan
2. Work Breakdown Structures by Norman, Brotherton, and Fried
I’ve found both of these helpful. Both cover the use of nouns and verbs in the WBS and schedule, and both discuss the advantages Ben mentioned in using nouns to clearly describe WBS deliverables and present-tense verbs to describe the detail schedule activities required to produce the deliverables.