Note: Do not waste your time on books covering bash expect for the very first start.
Even then - they're usually aimed at beginners or people who just need a general overview of the language functions, but usually aren't worth their money.
Half of the authors appear as if they couldn't even tell the difference between Bourne, POSIX and C shell.
An Info-Website for ksh.
Also check out the "memorandum for file" - the original ksh release notes. These 30 pages alone contain more knowledge than 90% of shell coding books.