Yep, there is really no other book like Harrell. There's a 2015 edition with some good improvements too. Definitely not an introduction, though – it assumes you already know how regression works and preferably have already used it quite a bit in practice. For an introduction, I might recommend Downey's Think Stats [1] for a gentle introduction, Kutner's Applied Linear Statistical Models for a more traditional, mathy introduction or Gelman and Hill's Data Analysis Using Regression and Multilevel/Hierarchical [2] for something in-depth but still very approachable.

[1] http://greenteapress.com/thinkstats2/

[2] http://www.stat.columbia.edu/~gelman/arm/

great points. it's not an intro to regression book. although Harrell's examples are very well laid out and easy to follow.