Can a Series Connected motor be run straight from batteries?

Of course ! Most electric vehicles(*) like forklifts etc are run on a battery and connected in series. Of course the motor has to be made for that. For speed and traction control often it is just a PWM (Pulse Width Modulator) that controls a thyristor who feeds the motor with longer or shorter pulses. But if you want it to be not as complicated, the speed/traction can be regulated by putting resistors in series with the motor. Very old forklifts etc work this way. You don't have a smooth variable speed, but the more resistors you put in series, the slower your motor will run. It feels like you have kinda gears.

(*) Newer models often are driven by a 3 phase AC motor, and get fed by an inverter (frequency control)

If you run a motor on DC do you still use a Thyristor to control its speed with? Is there a literature that I could read upon about this, especially on forklifts?

A series motor should not be considered to be anything like a permanent-magnet motor.

A series motor has a constant power characteristic which means it has enormous torque at low speeds but declining torque as the speed rises (torque x RPM = power). If the load is disconnected from a series motor, there is nothing to keep the speed from running away. Series motors are used as the starter motor in cars.

A permanent magnet motor has constant torque throughout its speed range but will not run away. Typical permanent-magnet motors are windshield wiper, power window etc. motors in cars that are expected to run as a predetermined speed.

With the appropriate controller, either type of motor can be made to work. You do not indicate whether your application is closer to constant torque or constant power and it would be best to select a motor closest to the characteristics of the application if you want to get away with the most rudimentary controller.