Yes, I believe it is El Capitan. With regards to books, are the books you recommend beginners books? They perhaps sound more like 'insider or hidden tips and trips for experienced users. I am not that.
As well as buying the iMac we did buy the MS Office suite for 5 users. Although it seems that I may be disappointed with it my actual use of office is really just limited to email and writing a few documents. I bought the licence for 5 machines since if our iMac experience is a positive one we are anticipating replacing our other windows laptops. What we are really hoping for in the future is a limited number of machines easily staying in sync with one another with minimal user intervention.