Really impressed by the international slide rules museum

[Helix]

I've recently fallen in love with slide rules! This is strange, because I've never used a slide rule at school, but I am fascinated by these beautiful instruments.
I already own nine slide rules, linear or circular…

Currently I'm learning the use of slide rules with "The Slide Rule" from Lee H. Johnson. This book gets excellent reviews on the following sites:
http://www.steves-sliderules.info/alltexts.html
http://www.sphere.bc.ca/test/books.html
http://www.sliderule.ca/books.htm
Its contains a lot of illustrations based on a real slide rule, and simple diagrams to explain the math behind calculations. Of course it is available on the International Slide Rule Museum.

I've also the book "Slide Rules, a journey through three centuries" from Dieter von Jezierski. It has a nice section on the history of slide rules, plus short articles on the major makers, with comments on some on their remarkable products.

My next book will be "La Règle à Calcul, la longue histoire d'un instrument oublié", from Marc Thomas. It's entirely devoted to the history of slide rules, and their role in the industrial development of our societies. Since this book is in French, there is a strong emphasis on French makers. This book should be available very soon.

Besides the ISRM site, it's worth exploring the site of the Oughtred Society: http://www.oughtred.org

About the number of digits necessary, it depends on the subject. On many physical problems, 3 digits are enough. On the other hand, for ray tracing in optics for example, a lot of digits are necessary, and the advent of computers was a huge progress in optical design.
For calculations with many digits, I think that before calculators, tables were used:
http://locomat.loria.fr/