In today's economy, one of the main problem for students is the cost of textbooks.

To combat this problem, I started the Potto Project (www.potto.org) two years ago in which two textbooks have been written. One of the books, Compressible Flow, also includes comprehensive software package. The experience of writing these books and the knowledge that was gained is described here.

There are a very limited number of open content college textbooks most of which in Mathematics and Physics.

There are less than 5 quality open content textbooks for engineering. Even if we broaden our definition of open content to a free book, there are still very few free engineering textbooks. So, why are there so few textbooks available?

The main difference between creating an open source software and an open content college textbook is that the later

requires a Ph.D. or equivalent. Open source software can be created by a talented individual(s) with some limited knowledge. For example, GIMP was started as a class project, and Linus Travels started to build the Linux kernel when he was a student. However, for any textbook project, the author needs a solid background and understanding in the field prior to start writing a textbook. There are no quality engineering textbooks that were written by someone who did not have a Ph.D. For example, see Wikipedia (students in Purdue) attempt to write acoustics book http://en.wikibooks.org/wiki/Engineering_Acoustics. The fact that a book is free is not enough to make the book successful.

There are some reasons that so few engineering Ph.D.s write open content textbooks.

It takes a year or more to write a good textbook. More importantly, however, the problem. that initially plagued

the open source, is the credibility. Many view the open content books without merit. As one student put it, ``If the book is free, it must be lousy.'' This statement isn't true for Kuphaldt's book (Tony Kuphaldt, Lessons In Electric Circuits see http://ibiblio.org/obp/electricCircuits). Nevertheless, perceived lack if creditability deters (or discourages) many possible writers.

The term "compressible flow textbook" searched by any search engine will show that my book is the most popular in the field. So what makes my book successful? Let me here be frank. I decided that the book should be written for the students thus I did not have a dilemma of who I am writing for (see Bauman's dilemma http://chronicle.com/free/v49/i43/43b00501.htm). Writing textbooks for students are different. They want material with examples that can help them to solve the homework, so I provided abundant examples. Students want to have the center and complicated issues explained in simple mathematical terms.

The central issue of compressible flow, the oblique shock was explained by a hand waving technique (why there is a detached shock etc.). I was fortunate to be able to solve it and explain it in simple mathematical terms.

Not only the main concepts but practical issues need to be tackled. For example, I took the issue of moving shocks that required a complicated procedure to solve and provided simple tables so that no complicated calculations are needed.

A common to the open source software, no matter how talented one is, the fact remains that power is with the mass.

Many have contributed, criticized and helped to improve the book. There is no way, one person can see all the issues.

With help of others mistakes are solved which leads to a better book.

Just as the open source movement took time to establish its credibility, so the open content textbook revolution has just begun. As opposed to open source (software), the open content movement (textbooks) requires convincing Ph.D.s to write the textbooks. There are simple things that author can do make their book successful, such provide numerous examples, simplify complicated mathematical issues, and seek help from others.

Most importantly, open content textbook should be written for the students and not for the instructors.