>>Why use textbook sources at all for compsci?

You raise some good points, but it isn’t that simple in most courses. On occasion I have skipped using a textbook and have substituted material I have prepared myself along with a bibliography of web sites and library references. But that does not work very often.

I will break the situation down into lower level and upper level courses and highlight the “no textbook” problem in each category. Please understand that these are very broad categories and that I am over simplifying both.

Lower level courses are often about specific languages or tools. These are things like Java, JavaScript, HTML, some flavor of SQL, or productivity software. There are abundant sources of online tutorials on all of these topics. Rarely is there a single, high-quality tutorial series that covers all of the topics we need to cover in a course. Most of them are task oriented and they rarely do the type of comprehensive coverage of a topic that we need to cover. You can try to stitch together several different tutorials to get a more comprehensive perspective on a topic, but every tutorial is written using different idiosyncrasies and with different assumptions about the user’s background. People can get themselves productive using on-line tutorials and reference books, but it takes a lot of time and a willingness to try, fail, explore followed by more searching and experimenting. We don’t have time for that in most courses.

Another problem with going textbook-free in lower level courses is that students want textbooks. Students in lower level courses don’t tend to regard the teacher as an expert, and they are often intolerant of the frustrations that come from ad-hoc resources. I’m tenured and have thick enough skin to plow through regardless of student attitudes, but lower level courses are often taught by young, inexperienced faculty. They often need the security of a textbook as much as the students do.

Alternative sources for lower level courses have a couple of things going for them that encourage development. They have volume and relatively consistent content requirements. Most CS-1 Java textbooks are going to cover the same basic material, and there are a lot of people out there who can use it. You have a lot of faculty members willing to contribute things like test backs and end-of-chapter problems that are important to textbooks but a pain to prepare.

Upper level courses are often more amenable to ad-hoc resources, but they often present their own problems. Upper level courses are more likely to be on broad concepts such as Compiler Theory, Operations Research, Operating System Design, or Security. Like lower division courses, it is often very desirable to have one central source of information that has some consistency throughout the course. However, historically upper division courses frequently reference other sources. The students have more maturity and technical experience, and they are much better at dealing with different perspectives and authors’s various styles.

There are some alternative upper level textbooks that are coming along nicely, but most lack comprehensiveness. My most recent experience was looking at Operating Systems textbooks. Most of the authors of the books only write chapters for content they cover in their courses. This leaves some big gaps for everyone else. There are some more comprehensive projects with contributions for multiple authors, but these are sorely lacking in cohesion. My hope for this type of textbook is that we will see some strong editors emerge who can put together a team of authors who will work toward producing unified and relatively comprehensive resources.