Retyping some material to obscure source of a textbook with a solution manual? This is my first time teaching mathematics. I've been giving lessons based on a certain textbook which contains excellent exercises (not just the exercises, but the order in which they appear leads one to discover mathematics for yourself). However, the textbook has a solutions manual, and I am certain that if the source was revealed, then students would purchase the manual and defeat the purpose of the course. I am wondering if it is ethically acceptable to retype myself some of the exercises and exposition from parts of the book in order to obscure the source (and perhaps reveal it at the end of the course)?

My intentions are solely to create a joyful mathematical experience for the students. There is no intention of plagiarism.  My only worry is that the exposition is so good that I don't see how much I can deviate.

Clarification: The course is not intended to have students master a technical subject and then solve problems under a time pressure.  This is not a mathematics course for engineers or scientists.  Rather, problem sheets are given each week, with the requirement that at the end of the week the student turns in 10 solutions to their favorite problems.  The focus is on quality of writing and clarity of though.  In fact this entire issue is now resolved as I have communicated with the author.
I can think of the following potential ethical issues, some more easily mitigated than others:


Misleading others into believing that you have created the exercises yourself - You can easily avoid this by explicitly noting that the material is adapted from another source (without specifying the source). In most cases, there is no expectation of originality in teaching materials anyway - see Is it considered plagiarism for a professor to use uncited sources in teaching materials?
"Stealing" professional credit or similar benefits that would otherwise be due to the original authors - If you are redistributing the materials to colleagues for use in their courses, or otherwise using the materials in a way that would earn professional "credit" or goodwill (not just assigning the exercises to students), then you should definitely cite the source. Otherwise, I don't see a problem here.
Withholding a potentially useful resource from the students - If the book is genuinely helpful, I don't like the idea of preventing students from finding out about it. (Surely there is some more content and other exercises in the book that you aren't rewriting and giving to your students, that they might benefit from?) If you judge that the educational benefit of preventing some students from finding the solutions is really much greater than the benefit of some students buying the book and using it to learn more effectively, fine; but think that through very carefully.
Suppressing sales of the textbook - Presumably if you did not obfuscate the source, some of your students would buy the book (because it is such a helpful learning resource), and the authors of the book want people to buy it (or download it, if they have made it available in that way). I suppose you could mitigate this by getting permission from the authors. 


Personally, I would prefer to tell students about the textbook, but remind them that if they don't actually do the exercises themselves, they will probably do poorly on the exams. If that was not an option, my second choice would be to modify the exercises so that students who had the original text and the solution manual would only be able to solve them if they understood them.