More than books
Exam Cram 2, and the Cybex book are good. O'Reilly has a ton of questions; Exam Cram2 had the most difficult test questions, IMHO.
However, more than books, nothing beats sitting down at a test system that you can afford to lose at anytime, not the system you just got set up and is doing everything you want.
As most books will tell you, the actual exams are nothing like the practice exams. The best way to pass the exams is become saturated with the material. For me, typing out commands, creating users and altering their shells and permissions on a test machine, trying to build a kernel, and so
on was the only way I could absorb the material.
Others have told me the best way to learn linux is get hold of GenToo or Linux from Scratch and begin at the very beginning. That is build an OS from a live disk, add only the hardware support you need, and build a linux system from the ground up. This makes your exam learning experience -- assuming you have a full-time job -- last a long time and consume your social life.
I started last January, and passed the second LPIC-1 exam in August.