If you're in telecom, you need this -- it's the Bible. It's the best in the business, and if you can't find it in this book, it can't be found. Network engineers need to live by this book.
This book is really useful for engineers or if you only want to learn the fundamental concepts of telecommunications. starting from the fundamental concepts, seeing the quality of service, the PSTN, networks, giving definitions of many concepts, etc. It gives a general idea of everything that would come to be the field of telecommunications. You can find the frequency ranges, talk about the baseband, show equipment diagrams like codecs. It proposes exercises and questions at the end of each chapter which I think is great.