Can a person learn martial arts from books alone?

It depends on exactly what your goals are and how good you want to be at whatever it is you think you can achieve.

If by martial arts you mean fighting: no, you can’t learn fighting of any kind from a book. You need a coach to correct the faults in your moves; then you need partners of different kinds and a coach to constantly correct your faults. The word ‘constantly’ is relevant.

If you mean learn a set of moves: yes you can learn some moves. You’ll be doing them badly, but you will be able to learn the moves. I know someone who did this with a book on karate in 1965. When they finally made it to a karate club, at least they knew the names of the moves. Everything else was wrong.

This, then, is the same question as, “Can I learn brain surgery from a book?”. Of course you can. You will be the least competent and most ignorant brain surgeon around, but you will know everything that can be learned from a book. That would be about 3% of it, probably.

Or - thinking about it - “Can I learn dancing from a book?”. Or singing. Or anything that involves skills that cannot be learned except by doing it with a teacher. How can you learn to sing from a book? Answer - it is completely and utterly impossible.

Books on martial arts have tremendous value to the experienced student, the researcher, the historian, the seeker after more information, and the student who would like to widen their horizons. For a beginner they are a nice introduction that hints at what can be learned from a good instructor, and no more.

What can I do to get fitter before starting to learn a martial art?

Now this is a great question. It is sensible, and logical.

If you want to ask it, as a separate question, you’ll get plenty of good answers. They will mention things like strength, suppleness, stamina. Things like working up to doing 3x3’s: a 30–40–50 tri-set of press-ups, sit-ups and squats, then repeat, x3. Some basic stretching. Work up to a half-decent time for 3 miles (run 3 miles in a good time).

These things are worth their weight in gold when you start in a gym as a newbie.

Learning brain surgery, singing, or fighting, out of a book?

No.

You can learn some aspects of certain martial arts alone, but you cant ever be good at martial arts without at the very least a sparring partner in my humble opinion.

Traditionally, you can't learn an "art" except with a Sensei or other Instructor, because having an instructor is part of learning an "art" in the formal sense.