5 days is more than enough for Havana. Do all that Joao recommended, plus I'd recommend a trip to a factoria to see cigars being made/learn the history and art. Do not spend more than 2 days at Varadero. The beach is beautiful and worth a trip but that's a tourist trap (where Canadians and Europeans have been visiting for the past 60 years while American's couldn't). You can see most of Havana in about 3 days so if you are there for a full week, I'd recommend 3 days Havana, one or two in Veradero, and a couple in Santiago or another city. If you are flying roundtrip in and out of Havana, then maybe a few there to start, a couple in Veradero to get some beach and relaxing in, then back to Havana for a couple.

You do not need to purchase a guide book; everything you need is online. I do recommend reading the biography on Che Guevara before going - it's very long but it gave me a true appreciation for the history of the city and island when I was there. I'd also recommend staying in an Airbnb vs. a hotel. In Havana, it will not be the greatest but you'll have a great local source who can point you in the right direction and show you some local spots, plus the hotels are owned by the government/Castros and are WAY overpriced for what you get.