I recommend the book In The Buddha's Words by BhikkuBhikkhu Bodhi. It is an anthology or selection of translated suttas from the Pali canon. It is also thematically and systematically arranged. You can look at the Table of Contents on the Amazon page I linked. It is 512 pages long, which is quite alright. PDF version here.
I quote from the Preface:
In an ongoing series of lectures I began giving at Bodhi Monastery in New Jersey in January 2003,1 devised a scheme of my own to organize the contents of the Majjhima Nikaya. This scheme unfolds the Buddha's message progressively, from the simple to the difficult, from the elementary to the profound. Upon reflection, I saw that this scheme could be applied not only to the Majjhima Nikaya, but to the four Nikayas as a whole. The present book organizes suttas selected from all four Nikayas within this thematic and progressive framework.
This book is intended for two types of readers. The first are those not yet acquainted with the Buddha's discourses who feel the need for a systematic introduction. For such readers, any of the Nikayas is bound to appear opaque. All four of them, viewed at once, may seem like a jungle—entangling and bewildering, full of unknown beasts—or like the great ocean—vast, tumultuous, and forbidding. I hope that this book will serve as a map to help them wend their way through the jungle of the suttas or as a sturdy ship to carry them across the ocean of the Dhamma.