There are several online piano courses for adults but there are also are piano course for adults that are music books and they have been around for decades.
I will not mention anything specific but I have seen quite a few of them over the years and my overall impression is that they are not that great. Some of them have existed for several decades so obviously they are not really horrible because if they were, they would have disappeared years ago. I would also like to mention that I cannot claim to have seen every adult piano course in print and therefore there might be some that are absolutely brilliant.
With adult piano course books, all the ones that I have seen teach a chordal approach. This is fine for those that want to learn a chord method, but if you interest is in learning to play classical piano, then these books are not really helpful at all.
I don’t tend to like any of the adult course piano books that I have seen for the simple reason they tend to expect a certain level of prior knowledge in book 1 and not every adult piano student has any prior knowledge.
Not only do these books expect a certain amount of prior knowledge but in my opinion, when it comes to learning new techniques or new information, they seem to introduce a few or even several new concepts at the same time. I don’t think this is reasonable. I believe it doesn’t matter how young or old a piano student is, I believe it is important to learn one new thing at a time because it allows for the student to thoroughly understand and learn how to play it before moving on to the next new concept. My experience is that regardless of whether I am teaching a young child or a senior, learning new things one at a time is always a good idea.
I know that some online piano courses are clearly good at what they do because otherwise they would not have been around for some years. I also realize that not everyone wants to learn this way. Many don’t want to learn online and would much rather stay old school and learn from a music book. If learning from a physical piano book is more to your liking, I would encourage you to at least consider using a piano course designed for children. It is my opinion that these piano books on the whole, are much more step by step than their piano courses designed for adults and therefore much better.
It is important to know what your choices are in order to pick the right way of learning to play the piano for you. Whatever you choose, whether it is online or using a physical book, make sure it is a method suitable for the genre of music that you want to play.