IDFX Book Suggestions
The IDFX exam covers competencies in:
programming and site analysis
relationship between human behavior and the designed environment
design communication techniques
life safety and universal design
interior building materials and finishes
technical specifications for furniture, fixtures, & equipment and lighting
construction drawings, schedules and specifications
professional development and ethics
The Interior Design Reference Manual is the most frequently recommended book to prepare for the NCIDQ exams. In previous years, it appeared to be one of the most comprehensive books in preparation for the exam. With the recent changes in the 2021 Blueprints, some additional books are being recommended. Typically if the content is included in the reference manual, the supplemental books go into further detail and explanation, which can be helpful for areas that aren't as familiar.
The books noted below are my recommendations based on teaching study group classes for the NCIDQ exam. They are not sponsored in any way and I do not receive any funding for my opinions noted.

1. Programming & Site Analysis
The Shaping Interior Space book covers the fundamentals of programming and analysis. The areas focusing on Understanding and Ideation are of particular interest for those studying for this exam.
2. Relationship between human behavior and the designed environment

There are two books that cover this content really well. You can select either or both. Human Factors in the Built Environment explains the relationship of the human body to the design process so that you can plan and detail interiors. Key topics include proxemics, anthropometrics, ergonomics, sensory components, environmental considerations, special populations (elderly, children, special needs), and universal (inclusive) design.

Health, Sustainability and the Built Environment covers similar topics to the Human Factors book, but it highlights the relationship of the built environment to sustainable initiatives. If you have limited exposure to sustainability in your design practice, this may be a good suggestion.
3. Design communication techniques

Design Drawing is yet another illustrative showcase by author Francis D. K. Ching. In this book he artfully guides the reader through the design process, step by step. His materials frame the foundation for any architect or designer to communicate effectively within the design community and with our consultants.
4. Life safety and universal design

The Codes Guidebook for Interiors is one of the most comprehensive sources in aiding to understand the codes. It thoroughly explains concepts with relevant examples and illustrations that drive home the point. The 7th edition covers the 2015 codes, but a new version will be published in Nov 2021 for the 2018 codes. Universal Design is covered the Human Factors in the Built Environment

An alternate resource for the 2018 codes is the International Building Codes and Guidelines for Interior Design. This book thoroughly covers the design process and code integration throughout all phases of design. There are several project examples and case studies from different sectors.
5. Interior building materials and finishes

Specifying Interiors covers codes, materials, finishes, fixtures and furniture used throughout the design process. It describes the basic requirements for installation, code restrictions, flammability standards, and specification samples for various products used in the design industry.

J.J. Pizzuto's Fabric Science is an in depth look at the textile industry. Portions of this book pertain to the fashion industry, but there are 3 chapters that highlight the functionality and uses of fabric in interior spaces and interior design. There are also detailed descriptions about the fabric flammability testing standards.
6. Technical specifications for furniture, fixtures, & equipment and lighting
You can also review the Specifying Interiors book noted above for specification samples needed for the IDFX exam.

Designing a Quality Lighting covers lighting in more detail than you need to know for the NCIDQ exam, but it also includes the wattage calculations, in depth explanations of the different lamp types and uses for each.

Building Systems for Interior Designers provides an in depth look at the infrastructure used in buildings to create the built environment. This book covers HVAC, lighting, acoustics, fire safety and more.
7. Construction drawings, schedules and specifications

Interior Construction and Detailing for Interior Designers & Architects is an in depth look at the different construction methods used in the industry. While the Interior Design Reference Manual includes excerpts from this book in it's chapters, this book covers the content more thoroughly. Dedicating a chapter to each topic, such as partitions, versus just a few pages in the reference manual. The detailed images used in this text are very useful when studying for any portion of the NCIDQ exams.
8. Professional development and ethics

The IDFX exam only covers a small portion of this book (2 chapters), but if you are studying for the IDPX exam as well, I highly recommend you add this to your library. Professional Practice for Interior Designers covers common elements needed to run your own design firm. Everything from contracts to human resources and hiring staff. This book is needed more for the IDPX exam than the IDFX, but the ethics and professional development and ethics sections are included in more detail than the Reference Manual.