Do you have books you MUST try to read almost annually?

Do you have books you simply must try to read almost annually?

I have several, but one stands out amongst them: Stand on Zanzibar by John Brunner.

As Joe Haldeman says on the cover, "It's time for a new generation to read it".

Another book (or, in this case, set of books) I try to read every year or so is The Lord of the Rings (but then again, who doesn't, right?)

So, I ask again: Do you have books you simply must try to read almost annually?

   

Update 11/15/2006 (Happy Birthday Jeff!): I just found this link, which mentions both Zanzibar & LotR.