tl; dr: Maybe, maybe not, but trouble seems likely.
Your text files are clearly derived from Dr. Seuss' books, so you normally need the permission of the copyright holder of the books.
You mention "fair use" as a possible way around this. However, I don't think this will necessarily fly. The releveant regulation says:
Notwithstanding the provisions of sections 106 and 106A, the fair use
of a copyrighted work, including such use by reproduction in copies or
phonorecords or by any other means specified by that section, for
purposes such as criticism, comment, news reporting, teaching
(including multiple copies for classroom use), scholarship, or
research, is not an infringement of copyright. In determining whether
the use made of a work in any particular case is a fair use the
factors to be considered shall include—
(1) the purpose and character of the use, including whether such use
is of a commercial nature or is for nonprofit educational purposes;
(2) the nature of the copyrighted work;
(3) the amount and substantiality of the portion used in relation to
the copyrighted work as a whole;
(4) the effect of the use upon the potential market for or value of the copyrighted work.
17 U.S. Code § 107 - Limitations on exclusive rights: Fair use
So the main factors are:
- the kind of use intended
- the type of work
- the amount used
- the effect upon the market value for the work
The kind of use (research) is probably not a problem, as is the type of work (a published book).
However, the amount used (several/all the books) would speak against "fair use". Using some small excerpts from the books may be okay, but using all of them probably not.
One mitigating factor: The effect upon the market could be argued to be small, since you will not publish the illustrations, and the Dr Seuss books are arguable quite incomplete without them.
In summary: You may get lucky with the fair use defense, but I would not count on it. Using the book texts for research yourself could be okay, but publishing it most likely is not.
The article Text and Data Mining and Fair Use in the United States addresses your problem, and mentions some court cases. The fair use defense was upheld in some cases, but none of these published complete works, only excerpts.