The English-speaking world may be divided into (1) those who neither know nor care what a split infinitive is; (2) those who do not know, but care very much; (3) those who know and condemn; (4) those who know and approve; and (5) those who know and distinguish.
1. Those who neither know nor care are the vast majority, and are a happy folk, to be envied by most of the minority classes. “To really understand” comes readier to their lips and pens than “really to understand”; they see no reason why they should not say it (small blame to them, seeing that reasons are not their critics’ strong point), and they do say it, to the discomfort of some among us, but not to their own…
4. Just as those who know and condemn the split infinitive include many who are not recognizable, since only the clumsier performers give positive proof of resistance to temptation, so too those who know and approve are not distinguishable with certainty. When a man splits an infinitive, he may be doing it unconsciously as a member of our class 1, or he may be deliberately rejecting the trammels of convention and announcing that he means to do as he will with his own infinitives.
—Henry Watson Fowler
A Dictionary of Modern English Usage
Last year, I read The Language Wars: A History of Proper English by Henry Hitchings. The book is a history of what “experts”, qualified and unqualified, have considered correct English over the centuries. The split infinitive is interesting because Hitchings points out that (i) usage of the split infinitive has gone in and out of vogue over the centuries (as judged by the literature of the different eras), (ii) references to Latin and other languages are misguided because these languages only have one-word infinitives that can’t be split in the first place, and (iii) English has other two-word constructs that are regularly split without people getting up in arms over the fact. A good book!
That sounds great. My library doesn’t have it, but now that I’m making some money, I will try to pick it up. I know there was a very big thing in the 19th century to try to make English as much like Latin as possible. It’s all so silly. But I have a huge fondness for 19th century British bourgeois obsessions. Two books that illustrate different aspects of this are Reinventing Shakespeare and The Rise of the Indian Rope Trick. I highly recommend both.