> Using software is not one of the exclusive rights of Copyright holders.
Copying is, and copying into memory is inherently necessary to use. (Of course, in some cases, copying may be fair use.)
> If I have a legitimate copy of the software I can use it,
If you can find a method to use it without exercising one of the exclusive rights in copyright, like copying, sure, or if that exercise falls into one of the exceptions to copyright protection like fair use, also sure, otherwise, no.
> Just like I don't need a license to read a book.
> Has this interpretation actually been upheld by any courts?
That copying into RAM, including specifically in the context of running software, is included in the exclusive right of copying reserved to the copyright holder except as licensed by them? Yes, the main case I am familiar with being MAI Systems Corp. v. Peak Computer, Inc., 991 F.2d 511 (9th Cir. 1993) [0]; note that for the specific context of that case (software that is run automatically when activating a computer in the course of maintenance or repair of that computer), Congress adopted a narrow exception after this case , codified at 17 USC § 117(c) [1], but that validates that in the general case, copying into RAM is a use of the exclusive rights in copyright.
> it is not an infringement for the owner of a copy of a computer program to make or authorize the making of another copy or adaptation of that computer program provided:
> (1) that such a new copy or adaptation is created as an essential step in the utilization of the computer program in conjunction with a machine and that it is used in no other manner
i.e. the owner of a copy of a computer program has the right to make more copies if necessary to use it (e.g. copy-to-RAM, copy to CPU cache) as long as they don't use those additional copies for any other purpose. That same section also gives you the right to make backups as long as you destroy them when giving up ownership of the original.
You have not violated copyright because things in your memory are not copies. US copyright law defines copies as material objects in which a work is fixed and from which it can be perceived, reproduced, or otherwise communicated, either directly or with the aid of a machine or device.
Your brain is a material object but the the memorized book in your brain cannot be perceived, reproduced, or otherwise communicated directly (I can't look at your brain and "read" the book) and we do not have any machines or devices that can extract a copy of it.
Copying is, and copying into memory is inherently necessary to use. (Of course, in some cases, copying may be fair use.)
> If I have a legitimate copy of the software I can use it,
If you can find a method to use it without exercising one of the exclusive rights in copyright, like copying, sure, or if that exercise falls into one of the exceptions to copyright protection like fair use, also sure, otherwise, no.
> Just like I don't need a license to read a book.
You can read a book without copying it.