Dear Rose,
I found Bhikkhu Bodhi’s “In the Buddha’s Words” very insightful and helpful at the beginning (available from Wisdom Publications or from Amazon, or…). There is also a ‘Reading Guide’ to this book here on discourse.suttacentral: In the Buddha’s Words
There is also Nyanatiloka’s “The Word of the Buddha” (available as a free pdf download or also as printed copy), which I enjoyed reading. However, Nyanatiloka takes up Early Buddhist concepts as well as concepts from the Abhidhamma without stating clearly which is which. The book is still very helpfull and inspiring to read. I also know that there is a series of talks by Ajahn Brahm, where he goes through the chapters of the book and discusses the main points in brief (he also clarifies which teachings are Early Buddhist Teachings and which are Abhidhamma). However, I would not know where these talks are available online for download just now - you might want to try your luck at the Dhammaloka website - see above links by Bhante Brahmali).
All the best for your studies and with much mettaa,
Robert