The quality of writing in the best books is such that a lot of the non-obvious issues are laid out before you even start. An author with a lot of experience will know to tell you about things like: unexpected side-effects, long-term repercussions, undocumented features, typical usage scenarios...

Basically, you are getting the benefit of a writer with a lot of experience walking you through the problem and pointing out the truly interesting bits.

Sometimes you can find this in online references as well, but because of the low barrier to entry a lot of the information online is too basic, missing on details, or just outright wrong. This can happen with books too, so make sure you only outright believe those books which are classics with consistently good reviews. Other books can also be useful, but only as one of the sources, you will still have to verify the information by cross-referencing or trying yourself.