I'm in a Woolf phase right now and came to this book after reading her later works: 'Mrs Dalloway', 'To the Lighthouse', 'Orlando' as well her non-fiction piece 'A Room of One's Own'. Although lacking the brilliance of these later works, 'Jacob's Room' is well worth a read if only to see Woolf starting out on the process of trying to dismantle the idea that novel = linear narrative, fully rounded characterisation, and an omniscient author.
I think 'Jacob's Room' is a very 'impressionistic' novel in that we get no concrete sense of who the main character, Jacob Flanders, is. As another reviewer has said, we only catch glimpses of him. There is no god-like omniscient author forever telling you what the character thinks or feels. (To me this reflects the increasingly godless modern world we live in.) Also, the dialogue in the novel is often disjointed and I think this reflects the atomised modern world we live in. I assume that by writing dialogue which lacks linearity and fluidity Woolf more truthfully reflects human to human interaction: it's often full of non-sequitors, fails to flow easily, is interrupted etc.
If you're prepared for a challenging read and want to see how Woolf got started on the process of challenging the then accepted norms of fiction wfiting - then buy this book.