There is no perfect text for this course. But there are lots of sources. I also understand that textbooks are expensive!
All source texts are either accessible online or via license through the the Georgia Tech library, with the exception of the text by Johnston and Wu.
While I find Johnston and Wu to be the best text in terms of explaining material in the first 4-6 weeks of class, on the course outline I will link to alternative texts available to you online that are either freely available or via license from the Georgia Tech library.
Hyperlinks are to the publisher's website or, where available, to the online version or listing page at the GT library that can facilitate electronic access (Georgia Tech login required)
What should you buy? That is up to your budget. If money is no object, the Johnston and Wu text is highly recommended as a text. Likewise, if you want a single comprehensive reference text for the future (terse and detailed, but not as explanatory as I'd like) I recommend the text by Plonsey and Malmivuo.
Besides the usual online sources (like EBay and Amazon.Com) a great source of used textbooks not know to many students is www.abebooks.com. This is an online index of the holdings of many used booksellers across the world. Graduate level references texts are often easy to find here.
Foundations of Cellular Electrophysiology, by D. Johnston and Wu. This text covers does the best job describing passive dendrites, cable propagation, and basic single neuron electrophysiology. It is the most important text for the first half of the course. For most of the material, similar material is described in some of the electronic texts linked below - I just find that this book does the best job!
Bioelectromagnetism, by R. Plonsey and J. Malmivuo. If you want a single book that covers everything in this course to some degree, this is it. It is terse, heavy, and has impressive breadth, but is ideal if you already know what you are looking for and need some technical details. The link takes you to an online version, and a zip file of the book can be downloaded. I also use this text for some of the whole heart cardiology information.
Bioimpedance and Bioelectricity Basics, by Sverre Grimnes and Ørjan Grøttem Martinsen. We only will spend 2-3 lectures on this material, but there is an entire text devoted to this topic for the interested. The relevant chapter in Bioelectromagnetism is likely sufficient for the class. This link to the GT library page has a link to an "online" version of the text. It is not an easy-to-use online version -- one page at a time can be viewed using the linked to interface.
Self-written material and journal articles. These will be linked to from the password protected section of the course outline page.
Quantitative Neurophysiology, by Joseph V. Tranquillo. This links to the GT library page, from which you can go to the publisher's site and download a complete PDF version of the book. I will provide links in the course outline to this text a lot, especially as an alternate to Johnston and Wu. This is also used for some material on stimulation and recording.
Neural interfacing: Forging the Human-Machine Connection, by Thomas D. Coates. This links to the GT library page, from which you can go to the publisher's site and download a complete PDF version of the book. We will primarily use the chapter on electrodes and interfaces.