There are several books available which are good for beginners.
The Australian Esperanto Association provides a very easy pocket textbook. An old but good book, Jen Nia Mondo, can be downloaded from the Esperanto Association of Britain.
Esperanto by Direct Method by Stano Marĉek: words are defined by accompanying illustrations and include the 750 most commonly used words in Esperanto. This direct method allows you to learn without ‘translating’ between English and Esperanto.
Esperanto in Fifty Lessons: A Practical Guide to a Working Knowledge and Command of the New International Language by Edmond Privat is available online at Book Depository.
Two books often available in bookshops and libraries are Esperanto – learning and using the International Language by David Richardson and Teach yourself Esperanto by Cresswell, Teach Yourself Books.