American Institute of Mathematics:

To gain our seal of approval an open source mathematics textbook must be able to serve as the primary text in a mainstream mathematics course at the undergraduate level in U.S. colleges and universities. That means that we are not evaluating instructional modules, Java applets, supplementary lecture notes, or other materials that are designed for limited use within a course. Since the minimum length of a traditional course is 10 weeks with 30 hours of class instruction, the books we evaluate must have enough material for that, and most will have more since most college courses are 14 or 15 weeks in length with at least 40 hours of class time.

We are looking for books that we can recommend for consideration to any of our colleagues, books that are suitable for use in traditional university courses. Books must be mathematically sound and written in standard English with evidence that they have been proofread and edited. They need not have color, flashy graphics, or ancillary materials. They must have exercises, and though it would be desirable to have short answers to many of the problems, it is not necessary for approval. It would also be desirable to have more complete solutions available—at least to an instructor.

Furthermore, we expect books of the quality we seek to be class-tested. They should have been used (and be in current use) by faculty other than the author. We are certainly interested in fostering the writing of open source books, and so we may encourage people to try out a book that looks promising, but we would not label such a book as having our full recommendation.