For instance I read all of "The Unix Programming Environment" by Kernighan and Pike. It's FULL of useful information (in fact I'd argue it's way better than reading man pages) and after I was done I felt like I was 10 times the Linux man I was. However unless you're fighting with Linux a lot on a daily basis you quickly forget 80-90% of what you read. For instance, I found it near impossible to remember what '@' does in bash scripts.
It's quite demoralizing - and I've had the same experience many many times with technical books. You not only have to read, but you also have to DO the things you read (strangely this isn't the case for history books, or books on the soft-sciences). This can be almost impossible for the more obscure (though still very useful) features like the zillions of cool-but-not-so-often-used flags you will come across in your man page reading.