Yet another Rhymes With Orange cartoon, mostly silliness, about tongue twisters:
There's tons of stuff on tongue twisters, most of it what I think of as "tongue twister appreciation": collections of them, sometimes in a number of languages, for the reader's enjoyment.
There's also a certain amount of technical literature about them, in particular some linguistic studies about the patterns that make people prone to the errors (of the substitution/transposition type) in certain expressions. The /s/-/ʃ/ alternations in "She sells seashells …" are somewhat troublesome, but far from the worst there is (as in things like "rubber baby buggy bumpers", which makes grave problems for almost all "normal" speakers, especially when these speakers are asked to repeat the expression).
As far as I know, speech therapists don't use tongue twisters as a teaching tool (though I could be wrong).