Here is a list of (mostly) Java CMS tools you could learn. I recently searched for some CMS tools and found this page useful.
Note that CMS tools are built on top of other frameworks, so you may have to explore many other technologies, not just servlets. There were also some similar questions posted on SO by people who were trying to get advice on choosing a CMS. You must check them out.
About textbooks, I can’t say. I looked for an already built solution and expanded it, but if you want to “disable” some of the open source CMS to see how they work, first check the amount of documentation available and the community (not all of them are fully documented).
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