Why do they need a handbook? Basically you'll wind up trying to get them to know information that's not relevant to their individual roles, responsibilities. Real confusing and waste of time...
Just get them to understand the importance and applicability of the EMS policy to them as an individual and how they can help fulfill the committments in it; the significant aspects and potential environmental impacts relevant to their individual roles (work/job); how they must individually respond to environmental emergencies; operational controls they must use to perform their roles (work/job); how they can access documents and records when and if they need them and how to tell if the one they're looking at is the correct one; how they can communicate relevant environmental information to whoever needs it; and that's pretty much it.
The average employee doesn't need to know diddly about management review, the determination process of significant aspects, how legal requirments are identified, etc....unless it is relevant to a role they play and the responsibilities they must meet.