People tend to lump "Alice's Adventures in Wonderland" and "Through the Looking-Glass (and What Alice Found There)" into one collection which has taken on the new title of "Alice in Wonderland". This is probably a product of the movies, which took bits and pieces from each and made a composite adventure. This was possible, because Lewis Carroll (a.k.a. Reverend Charles Lutwidge Dodgson) managed to make the stories so even in quality that they can be put together seamlessly. He also managed to keep the stories enough different, that one can still enjoy reading both of them one after the other, without the feeling that the second is just a retelling of the first.
To be sure, there are several ways in which the stories are similar, but not to the point where it detracts from the reader's enjoyment of the story. There are only three characters which appear in both books, one of which is Alice. The other notable characters (the Cheshire Cat, the Queen of Hearts, Tweedledum and Tweedledee, Humpty Dumpty, etc.) are well distributed between the two books. Thus there is a looking-glass between the two, just as the looking-glass plays such a key role in the second book.
The Penguin Classics edition of "Alice's Adventures in Wonderland and Through the Looking-Glass" includes both books including the illustrations by John Tenniel. It also includes the original "Alice's Adventures under Ground" which includes Lewis Carroll's artwork. For additional features, it includes `"Alice" on the Stage' an article which Lewis Carroll wrote after seeing a production of the stage version, and it includes preface's to the books which Lewis Carroll wrote in 1896 for the 1897 editions. There are wonderful notes for both books, and a very informative introduction by Hugh Haughton. There is other supporting material as well. To sum up, this edition has pretty much anything one could want, other than a complete collection of Carroll's work.
A last comment on the introduction, it covers the biographical information for Reverend Dodgson, and the information on how the stories came about. Some of this information may detract from one's enjoyment of the story, but one can certainly understand the decision to include it for those who are interested in Reverend Dodgson and his life. All in all, this edition is packed with everything and will suit those who just want to read the stories as well as those who want to delve deep into their origins.