There's nothing quite as awesome as getting hooked on a great book series. You suddenly realize there's a huge pile of content just waiting for you to love it. But starting in on a series, even one that's popular, can be intimidating. It's not always best to start at the beginning. Sometimes a series takes a few books to really get good. Other times, the best book for introducing new readers ends up somewhere in the middle. So where do you begin? The following are cases where I personally think you're better off starting somewhere other than the first book.
Wagers of Sin by Robert Asprin and Linda Evans. The second book in the Time Scout series, Wagers of Sin is a clear-cut case of the "Empire Strikes Back Effect," where an already-good story has a sequel that outshines it. The loveable rogue Skeeter Jackson (a minor character from the first book) takes over as protagonist in this amazing tale of a time-traveling con man. It's much more likely to suck you in to the Time Scout universe than the titular first book.
Neutron Star by Larry Niven. "Known Space," where Ringworld takes place, is a vast and beautifully detailed universe. The short story collection Neutron Star gives you an excellent cross-section of Niven's complex world. By reading Neutron Star first, you can enjoy Ringworld all the more because it will take place in a familiar setting.
The Player of Games by Iain M. Banks. The first of Banks's iconic Culture books, Consider Phlebas, is uncharacteristic of the series, featuring the normally peaceful and benevolent Culture in a brutal war. The Player of Games shows what the Culture is really like and how they interact with other civilizations. By reading Consider Phlebas later, you can truly appreciate what a shocking event the war was to their civilization.
Small Gods by Terry Pratchett. Nowhere near the beginning of Pratchett's rich and hilariously funny Discworld series, Small Gods is nevertheless a standalone novel that demonstrates one of the core concepts of its world: how belief shapes the gods. That, plus the excellent story and brilliant narration, makes Small Gods the ideal gateway into these wonderful books.
I, Robot by Isaac Asimov. R. Dneel Olivaw is Asimov's most commonly recurring character, and newcomers to Asimov's oeuvre might be tempted to start with the first book in which he appears, The Caves of Steel. This would be a shame, because not only is I, Robot one of the greatest science fiction books of all time, it clearly and completely lays out the Robots series to come.