Perhaps you should try some spelling variations:
"serie letras la clase." » "serie las la clase.", "serie tras la clase.", "serie letras y clase.", "serie letras en clase."
"serie letras de clase." » "serie las de clase.", "serie detras de clase.", "serie letras de clacso."
"serie lecturas de clases." » "serie lectura de clases.", "serie lecturas de bases.", "serie lecturas de clacsos."

Try widening your search to All Fields.

You may be able to get more results by adjusting your search query.

• Performing a fuzzy search may retrieve terms with similar spellings: (((("serie letras la clase.") OR ("serie letras de clase."))) or ("serie lecturas de clases."))~.
• If you are trying to use Boolean operators, they must be ALL CAPS: (((("serie letras la clase.") OR ("serie letras de clase."))) OR ("serie lecturas de clases.")).
• The words AND, OR and NOT may confuse the search; try adding quotes: "((((\"serie letras la clase.\") OR (\"serie letras de clase.\"))) or (\"serie lecturas de clases.\"))".
• Removing quotes may allow a broader search: (((( serie letras la clase. ) OR ( serie letras de clase. ))) or ( serie lecturas de clases. )).
• Adding a wildcard symbol may retrieve word variants: (((("serie letras la clase.") OR ("serie letras de clase."))) or ("serie lecturas de clases."))*.