The world of aircraft engineering is truly remarkable, and even the smallest components involve a wealth of knowledge. I'm eager to explore this field and embark on the journey of building an aircraft. However, I find myself overwhelmed by the sheer volume of available information in books. Could you kindly guide me through the steps and processes I should undertake to start the journey of constructing a small 9-12 seater aircraft? Any recommendations for specific books or resources that could assist me in this endeavor would be greatly appreciated
It's not possible to solo-develop a 9-12 seat airplane to a legally flyable condition. The authorities want passenger aircraft built by people who know what they're doing, then checked again by another set of experts. For a 9-seater, the certification paperwork alone will take more time than a single human's lifespan.
Bjorn Ferhm, an aerospace engineer and a test pilot, has done a brief 34-part series of articles on what it takes to develop a small commuter airliner. He speaks from experience, and it's split into easily readable sections.
Part 5 covers the certification rules. A 9-seater will be FAA Part 23, Level 3. Anything more than 9 seats will be Level 4. The rest of the articles covers exactly the kind of airplane you envision. I strongly suggest you read through the whole 34-part adventure - it's what you'll do when designing the aircraft. Navigation is a bit challenging in there, but you can find it all browsing by author.
Here's a simplified to-do list to get a plane like that into the air.
You could skip a few steps for a one-off, but most are still necessary.
These articles won't tell you how to do all these things, only what to do. For how to do it, Daniel Raymer's Aircraft Design: A Conceptual Approach, widely available online, is the perfect start. It will get you to being able to create a brochure with a realistic aircraft proposal.
You'll need at least a dozen more books on aircraft design to finish the plans, and as many to manufacture them. Realistically, the easiest way to get it done is to study aerospace engineering in a good college, working with your fellow students. As a class, you'll often have a chance to design and build a manned aircraft prototype over the course of your 4-year degree. It will usually be a 1- to a 4-seater, but it will be the experience you need to go further.
The good news is: the rules for 1-seat experimental ultralights are very relaxed compared to anything else. You can build almost anything you want, as long as it flies reasonably low, slow, and doesn't carry anyone else. Specialized knowledge/education is still strongly advised in order to succeed with a new design, but it's possible solo.