There are two types of books that you need for the HVAC & Refrigeration PE exam. You need study material and references. Study materials include practice exams and study guides that will help to teach you the material necessary to pass the PE exam. References are books that contain equations, tables and large amounts of data in all sorts of topics related to HVAC & Refrigeration. I will go through the books that you need in both study material and references that are either free or cheap.

You should spend only around $320 to $400 TOTAL on study material and references. I don’t think that you should spend $1,000s on material that you will only use once in your life.

NCEES HVAC & Refrigeration Sample Exam ($53)

The sample exam provided by NCEES is the best way to figure out the type of problems that will be on the exam. You can also get a sense for the level of difficulty. The cost of this book is $40 and you get 80 problems. But this cost does not include shipping. The total cost with shipping to most places in the US is around $53. If you buy this book on Amazon with shipping, then the cost is roughly $70. You can check EBay for this book, but at the time of this post I was unable to find this book on EBay.
Link: https://account.ncees.org/exam-prep/340

NCEES HVAC & Refrigeration Outline (Free)

The outline should be used to help guide your studying of the following materials and references. As you go through the material, you should only focus on the topics that are within this outline. This will help to save time. For example, you do not need to know complex math topics, fluids, ethics, etc.
Link: http://ncees.org/wp-content/uploads/PE-Mech_HVAC-Apr-2017.pdf

NCEES Mechanical PE Handbook

This handbook will be your companion to the computer based Mechanical PE exam. Please remember that this handbook has the references for Thermal & Fluids, Machine Design & Materials and HVAC & Refrigeration. This means that the entire handbook will not be used on your HVAC PE Exam. In order to get this PDF for free, login to your myncees account. The link is below. Then go to Useful Documents then go to Reference Handbooks. Here you will be able to download the Mechanical PE Handbook. You should be very familiar with all of the contents within this book.
Link: https://account.ncees.org/login

Engineering Pro Guides Technical Study Guide ($45)

This technical study guide takes you through the key concepts and skills necessary to pass the PE exam and also has some problems. The guide follows the previously mentioned outline and is very comprehensive. I have used the HVAC & Refrigeration guide and it was easy to understand and I also used it in the actual exam. You will have to find a way to print it out, because it is only PDF, but it is also very cheap at only $45 for around 500+ pages of specific content for the PE exam.
Link: http://engproguides.com/pe-mechanical-hvac-and-refrigeration-practice-exam-guide.html

Engineering Pro Guides Full Exam ($42)

You will need more problems besides the NCEES sample exam to practice and to hone your skills. Engineering Pro Guides has the cheapest exams. It is $40 and has decent reviews and you are also able to ask questions from the author. Other exams cost around $59 to $100. The Engineering Pro Guides are closer in difficulty to the NCEES sample PE exam. The other exams are more difficult. You can also search for free exams online that do not come with solutions. Other ways to get more problems are to do the problems within the references below. Make sure to focus on the topics in the NCEES outline when you do those problems too.
Link: http://engproguides.com/pe-mechanical-hvac-and-refrigeration-practice-exam.html

ASHRAE Fundamentals from EBay ($60)

The ASHRAE Fundamentals is one of the best references to be used on the HVAC and Refrigeration PE exam. It contains many of the properties you need for air, water and steam. It contains a lot of formulas and equations for the topics on the NCEES outline like fluid mechanics, heat transfer, thermodynamics, pressure-enthalpy and the vapor refrigeration cycle. This book will also help you to solve the problems on pipe and duct design. On Amazon, this book is around $200, but you can get older versions for much cheaper like this one below from EBay. You may think that you need the newest and latest version to pass the exam, but in my opinion you don’t. Just think about all the other people that passed the exam previously with older versions of this book. Engineering and science is not going to change drastically, all the formulas, equations and material properties are still the same.

Please make sure that you do not buy the SI version or the CD version. The newest version is 2017 and the versions are updated every 4 years. So you should get the 2013 version but the 2009 version will work as well.

ASHRAE Systems and Equipment from Ebay($50)

Similar to ASHRAE Fundamentals, the ASHRAE Systems and Equipment book is updated every 4 years. The latest version is the 2016 version and it sells for around $200, but you should get the 2012 version. This version is being sold on EBay for around $50. This book will help you solve the problems on cooling towers, boilers, heat exchangers, condensers, pumps, compressors, fans, expansion valves, accumulators, air handlers, enthalpy wheels, heat pipes, economizers, etc.

ASHRAE Refrigeration from Ebay($80)

This book will help you to solve the problems on the refrigeration of food, food storage, cooling and freezing times of food.

Websites

The last set of references include the websites that apply to the NCEES outline topics that are not covered in any of the ASHRAE books above.

Psychrometric Charts

Please go to your NCEES Mechanical Handbook for the applicable Psychrometric Charts at various elevations.

Economics

In case you encounter an economics problem with salvage value, present value, return on investment, etc. then you will need the following interest rate tables.
Link: http://global.oup.com/us/companion.websites/9780199778126/pdf/Appendix_C_CITables.pdf
Link: http://www.webpages.uidaho.edu/~mlowry/...

Cooling Towers

The ASHRAE Systems and Equipment book should have a lot of information on cooling towers, but in case you need more, please use this link.
Link: http://pdf.directindustry.com/pdf/spx-cooling-technologies/cooling-tower-fundamentals/34010-40110.html

Dampers & Control Schemes

There isn’t much information on dampers in the ASHRAE books but this website has a lot of good information on controls and dampers.
Link: https://customer.honeywell.com/resources/techlit/TechLitDocuments/77-0000s/77-e1100.pdf

Grills, Diffusers & Terminal Units

Titus is the best place for free information on any sort of terminal devices. This pdf shows the necessary information on grills & diffusers, comfort, throw, etc.
Link: https://www.titus-hvac.com/file/...