Molles, M.C. 2010. Ecology: Concepts and Applications. Fifth edition. McGraw-Hill. New York, NY. p. 572.
I'd say this is the "standard" undergraduate textbook. Those planning on continuing to graduate work should best know this book cover to cover by the end of their 1st year of graduate work.
Tons of pictures, lot's of definitions (with glossary), tons of references, and easy to read (as far as textbooks go).
Begon, M., C.R. Townsend & J.L. Harper. 2006. Ecology: From Individuals to Ecosystems. Fourth edition. Blackwell Publishing Ltd. Malden, MA. 738 pp.
As @fileunderwater said, this is the standard text for ecology. I'd say anyone moving through graduate work should know the concepts in this book front-to-back by the time they finish their graduate degree.
Strengths: Numerous relevant publications; good discussion; most in-depth/comprehensive of those I've read.
That being said, it's not the greatest place to start. There are no colorful pictures, there are no bolded definitions (and no glossary), and the discussion assumes you already have a grasp of simple concepts.
J Gurevitch, S.M. Scheiner & G.A. Fox. 2006. The Ecology of Plants. 2nd ed. Sinauer Associates, Inc Sunderland MA. 574 pp.
This is a great textbook to learn about the ecology of plants (well illustrated, lot's of definitions, fairly easy to read but still fairly comprehensive).
This book would best be read having already taken an intro ecology class.
Smith, R.L. and T.M. Smith. 2001. Ecology & Field Biology. 6th Edition. Benjamin Cummings, USA. 843 pp.
- This book is very dated, which results in a number of more recent studies not being mentioned. However, if the goal is to develop a simple understanding of ecological principles, this book could be found very cheap online.
Bush, Mark B. 2003. Ecology of a Changing Planet. 3rd edition. Prentice-Hall, Inc, New Jersey. 477 pp.
- This book is likewise very dated, which results in a number of more recent studies not being mentioned. However, again, if the goal is to develop a simple understanding of ecological principles, this book could be found very cheap online.
I would also say that the chapter introductions (as well as copied papers) in Foundations of Ecology (Real & Brown 1991) are great for catching you up on both the history of the field of ecology and the most important classic papers/researchers in the field.
Books on Communities & Succession:
These are a bit more dated, but here are some of the classic books on these topics.
Bazzaz, F. A. 1996. Plants in Changing Environments: Linking
Physiological, Population, and Community ecology. Cambridge:
Cambridge University Press, 320 pp.
Bormann, F. Herbert, and Gene E. Likens. 1979. Pattern and Process in
a Forested Ecosystem: Disturbance, Development, and the Steady State
Based On the Hubbard Brook Ecosystem Study. Springer-Verlag, New
York. 253 pp.
Glenn-Lewin, David C., R K. Peet, and Thomas T. Veblen, eds. 1992.
Plant Succession: Theory and Prediction. Chapman & Hall, London. 352
West, D C., Daniel B. Botkin, and Herman H. Shugart, eds. 1981.
Forest Succession: Concepts and Application. Springer-Verlag, New
Whittaker, Robert H. 1975. Communities and Ecosystems. 2d ed.
Macmillan, New York. 385 pp.