Legal Question in Employment Law in California

What kind of history book?

You need to repost your question with more information, starting with why the employee is bringing in the books at all, and whether or not the books are proper history books or racist screeds. You can't discipline an employee just for bringing legitmate books to work, if bringing books to work is legitimate, that happen to have symbols on them. Inside the Third Reich, for example, is an award winning non-fiction work by Albert Speer that in most editions has a swastika on the cover.

The context needs to be explained, but if the material offends employees and it bears no relationship to work, employer action probably should be taken. Consult with an employment attorney in your area to explain the circumstances what discuss what action may be appropriate.