The main reason for our visit to Devils Tower was to visit with Frank Sanders. One of the most generous souls I have ever met in the climbing industry and a must person to meet if you get the chance. He is a contributer to summitpost and maintains the Devil's Tower page.
We had the entire tower, Durrance route, etc, to ourselves on a great day in April, summit as well. Maybe two other parties on the entire tower! April, mid week, the way to go. 70m was perfect to rap the routes we did. There are stations all over this tower. If you are willing to be a bit tough and climb when it is a bit colder, you too can experience it as we did.
Yes on thunder showers, yes on being damn hot in the summer.
You do not need a guidebook. This is not a big place. Everything within the grade you are talking about is well described on line by Frank and others.