I totally understand . . . I feel like I'm pretty well-read, but then I hear other people mention authors I've never even heard of and I feel quite stupid!
From the list you've given, I've heard of Richard Dawkins, Tolstoy, Nietsche, Marx, Salinger, Orwell, Rosseau, and Ayn Rand. Dawkins, from what I hear, is about science, so I wouldn't call him necessary. Nietsche, Marx, and Rosseau are all about philosophy, so reading them isn't necessary, but a knowledge of them and their basic assertions is always good. Orwell I'd say is someone you must read, and Salinger too (although I never found much greatness in him). Tolstoy and Dostoevsky are wonderful fiction writers that I'd encourage you to read.
But probably the best way you could spend your time is with Ayn Rand. Seriously- I feel like every person should have read "Atlas Shrugged" by the time they're 20. It doesn't matter if you agree with her or disagree or think the book is utter rubbish- it's simply the kind of work that needs to be read and known and discussed. It will open your mind (it did mine) and you'll be key on a very big part of history- including the philosophy that at one time guided Alan Greenspan, and continues to have many other followers. I just think it's kind of big, and it is enjoyable reading, in my opinion (better than Tolstoy, at least). Give it a try!
And for me, I don't often read a book straight through, or in big chunks of time, unless it's absolutely brilliant and I'm spellbound or very easy and light. I read all through the day- in the bathroom, while eating, before bed, before class starts- little snatches that seem to get it read. That way I don't get burned out on a book and keep it fresh in my mind. But that's just me.
As for knowing words, I generally don't look it up in a dictionary, unless I'm completely stumped and it's crucial to my understanding of the text. I try to discern it as best I can, and if it isn't necessary, I leave it be.
Have fun reading!