Is it really worth buying this book if you already have the LG bible?
I doubt it. Get through the bible first and then see how you're scoring. If you find you're getting perfect or almost perfect then I wouldn't bother. If there's still room for improvement, you may find it useful.
The issues that I have are primarly related to time, and taking contrapositives/ related inferences into account.
Not sure what level you're starting from, but here's what I found helpful:
-Write out any rules you can AND their contrapositives (use symbols which are easiest for you to understand of course)
-Add as many rules to your sketch as you can - including entities that cannot go in a certain group or slot
-Make as many deductions as you can BEFORE starting the question (usually, there are certain exceptions to this rule)
-Use prior answers (acceptabilty questions can be very useful for eliminating answers in subsequent question) -- this is risky, but if you are confident the answer is right, it can save you a lot of time
Timing issues can be solved with lots of practice. In the long run, spending extra time making sure your set up, rules and deductions are correct can save you lots of time with the questions (and help you avoid silly mistakes). Once you've done MANY MANY sections, I don't think timing will be an issue. I must have spent 50+ hours learning logic games. I'd redo ones I had trouble with until I got them perfect. I went from getting 30% right to 95-100% right. It takes lots of practice but they can become your easiest section. Hope this helps. MSG me if you have any questions.