Here the 1874 RLDS edition accidentally omitted the preposition of after the verb know. It was restored to the RLDS text in the 1908 RLDS edition. The verb form “to know of something” is quite common in the text. Consider, for instance, the following similar examples where of could theoretically be omitted:
There has been some tendency in the textual transmission to omit the of from “to know of something” (besides here in Alma 17:16):
On the other hand, there is no evidence for accidentally inserting of after the verb know. In any event, either reading with or without the of is possible in Alma 17:16. The critical text will follow the earliest reading by maintaining the of in this passage.
Summary: Maintain in Alma 17:16 the of in “to know of the plan of salvation”, the earliest reading; such usage is found elsewhere in the text.