Which textbooks would that be? Are you aware that "Darwin's Finches" was published quite recently?
The book I am referring to was published in the 1980's, it won a Pulitzer prize. Hardly a book that has received no notice. Therefore the continued claim that they are different species in textbooks is thoroughly despicable and a lie.
However, the original claim that Darwin's finches were separate species was made way back in the 1930's with as I said the claimants not bothering to check if they could mate with each other and the evolutionists repeating the lie without question profusely in textbooks after that. It shows the total dishonesty of evolutionists as well as the total lack of scientific inquiry in so called evo-science.
Unless my memory is failing me, the observers who wrote "Darwin's Finches" made quite an extensive study of all the breeding going on in the Galapagos, including of crossbreeds, and their conclusions were not that the crossbreeds were more highly successful than the purebreeds, contrary to what you've suggested.
As I've already pointed out to you, speciation is largely a distinction of degree, not class. Being able to interbreed marginally is not an empedement to being properly classified as a set of separate species. The distinction is an arbitrary, artificial human one with no distinct, easily recognized, universally accepted boundary.
Are teacup poodles and mastiffs of the same species or distinct species? Depends whether you ask DNA researchers and en vitreo specialists or paleontologists and dog breeders.