Keep in mind that solving a case is a skill, not knowledge. You can read all the books you want, but eventually they are only tools to build the skill through practice. In my eyes there is no absolutely superior book, so if you've read some already, now all you need to do is get the practice.
For that, the free practice cases with peers are fine, usually until a handful of cases before the interview. You should then make sure to get professional feedback from experienced interviewers. Either friends that work in the consultancies or coaches, e.g. here on Preplounge.