Honestly, I don't think you can go wrong with any of her books. I know a lot of people start out with The Opinionated Knitter which is a compilation of some of her newsletters published in book form. I have that, The Knitter's Workshop (which you can also get in DVD form with Elizabeth herself leading the lessons) and the Knitter's Almanac which is a fun compilation of projects that you can knit from month to month.
All her books are very chatty and include patterns, some of which are repeated in other books. The Baby surprise sweater is included in both the Knitter's Workshop and The Opinionated Knitter.
Good luck in choosing!
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Well, I've wrestled with reality for 35 years, Doctor, and I'm happy to state I finally won out over it.
--Elwood P. Dodd