There's some characterization established in books one and two, and you miss out on some of the developments between Harry and Susan, Murphy, Marcone, and Morgan, but if you're having that much trouble with it, go ahead and skip it.
I'd suggest checking out a good summary of the plot of the two, though, as it will help you with some things that'll seem to come out of nowhere if you don't have at least a vague understanding of what goes on (like why Susan decides to (spoiler) in Book 3, for example)...
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Sucrose Octanitrate.
Proof positive that with sufficient motivation, you can make anything explode.
I'd suggest checking out a good summary of the plot of the two, though, as it will help you with some things that'll seem to come out of nowhere if you don't have at least a vague understanding of what goes on (like why Susan decides to (spoiler) in Book 3, for example)...
--
Sucrose Octanitrate.
Proof positive that with sufficient motivation, you can make anything explode.