Unfortunately most scheme books are dead tree only and many are out of print, though available in the used book market.
The classic is Structure and Interpretation of Computer Programs by Abelson and Susan.
It is probably your best option and is still in print. There is also an online version of SICP at https://mitpress.mit.edu/sicp/full-text/book/book.html.
One of the advantages of Scheme, being so different from what your students already likely know, is that you don't have the problem of finding a good book for "scheme as a second language" as you would switching from one language to a similar one. A book that assumes the students don't know a lot is fine for a paradigm switch situation.
There are other books though. Scheme and the Art of Programming by Springer and Friedman is pretty good. Friedman also published The Little Schemer, which is probably too simple for your uses. But combined with his follow on The Seasoned Schemer you might do ok. But Abelson and Sussman is the gold standard.
Haskell, on the other hand, being more modern, would offer the possibility of e-books better than Scheme would, if that is all-important.
As user ctrl-alt-delor mentions below, there isn't much to the Scheme language, so a "reference" fits on an index card. The problem with functional languages for those that program in other languages is that you need to expand the way in which you think to use them effectively. So SICP, while not a reference, as such, will help your students make the mental switch necessary. It isn't like needing a Java reference if you are a C# programmer. It is nothing like that at all.
Here is a page of interesting links about Scheme and how to use it.
In particular, there is a link to another page showing how to set up emacs for scheme. There are also links to instructor materials, such as exercises.