Thanks to the Internet, today's learners of Esperanto have options far beyond mine when I was teaching myself Esperanto in the ancient days of print culture a half century ago. I have not checked them out, but the place to go, I believe, is
lernu.net. In the ancient times I mentioned, one could order textbooks of various vintage from an Esperanto book service or borrow some from the public library, some of which reached back to the beginning of the 20th century. I tried them all, and none were too inspiring.
However,
William Auld tried a fresh approach, capitalizing on Esperanto's agglutinative character and acclimating the student to it, which is important in streamlining the future Esperantist's grammatical style instead of clumsily reproducing one's English manner of expression in Esperanto. I really liked this little textbook, but I loaned it to someone in 1976 and never saw it again. It is long out of print. I've been requesting online that someone scan it. Thanks to Enrique Ellemberg, here it is!
Esperanto, A New Approach
by William Auld (2nd ed.: Bruselo: Heroldo de Esperanto, 1969).
I seem to remember owning a copy with a plain blue cover, but I will never know. In any case, I got two bonuses from this textbook:
Soneto 18 de William Shakespeare, Esperantigis Reto Rossetti
Night of Summer (Somernokto) by Kálmán Kalocsay, translated from Esperanto by William Auld, with the Esperanto original
Auld created another introductory learning course, in a format called a Scriptographic booklet (which existed for entirely different topics):
A First Course in Esperanto (Greenfield, MA: Channing L. Bete Co., 1972)
Then there is Auld's successor to
Esperanto, A New Approach, an intermediate reader with exercises:
Paŝoj al Plena Posedo (Bruselo: Heroldo de Esperanto, 1968)
This book underwent revision with later editions; I don't know offhand which is the online text linked above.
Enrique has provided a spectrum of learning and reading materials on his web site:
Esperanto Fremont