I'm new to Esperanto and starting to use transitive verbs. The book I'm using, A Complete Grammar of Esperanto (Ivy Kellerman, 1910), states that "a transitive verb expresses an act of the subject upon some person or thing". It also states that "the person or thing acted upon is called the direct object of a transitive verb, and is given the ending -n". This seems to imply that the direct object has to be a noun.
However, Plena Ilustrita Vortaro de Esperanto gives the example sentence "mi rigardis tra fenestro de mia domo" here. Since "rigardi" is a transitive verb, the prepositional phrase "tra fenestro de mia domo" must be acting as the direct object. So it seems valid to use a prepositional phrases as the direct object.
And just for comparison's sake, if the sentence was instead "mi rigardis", it would be invalid because "rigardi" is transitive. And if the sentence was instead "mi rigardis la ĉevalojn tra fenestro de mia domo", the direct object would now be "la ĉevalojn" instead of the prepositional phrase.
Could someone provide an official or a credible source which explicitly outlines this grammatical rule? I'm trying to learn Esperanto with a firm understanding of all the constructed grammatical rules.