The Triathlete's Guide to Training with Power - Phil Skiba - Great book.

Jack Daniels Running Formula - Jack Daniels

Training and Racing with a Power Meter - Andrew Coggan, Hunter Allen

Swim Smooth - Paul Newsome, Adam Young - Also a great book.

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Chris Thornham
Co-Founder And Previous Owner Of FLO Cycling

Better Training For Distance Runners - Peter Coe and Dr. David Martin
Run Faster - Brad Hudson
Science of Running - Steve Magness

And a +1 for a number of the inspirational type books already mentioned

http://ironstruck.com/

One that hasn't been mentioned yet is "Run or Die" by Killian Jornet. He talks about how he started running, his epic run across the entire Pyrenees range, his record breaking Tahoe Rim run, and more.

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Paul
Current Top 2:
7 Things Non-cyclists Should Know About Road Cycling
The GORUCK Challenge Ruined My Life