Otdfbt

I'm a bit fuzzy on how the "personal a" works in Spanish, for example in this passage from Harry Potter y la cámera secreta:

Nunca se había sentido tan solo. Antes que ninguna otra cosa de Hogwarts, antes incluso que jugar al quidditch, lo que de verdad echaba de menos era a sus mejores amigos, Ron Weasley y Hermione Granger.

I'm new to echar de menos too, so my understanding is that Harry is the subject of echar, and lo que is its direct object.

I'm a little surprised you wouldn't use a lo que here rather than just lo que, considering the a that follows. Would a lo que also be grammatical? What about if you reordered the clauses to A sus mejores amigos, Ron Weasley y Hermione Granger, era (a?) lo que de verdad echaba de menos.?

A New Reference Grammar of Modern Spanish gives a related example: A lo que yo temo es a la maldita casualidad.

Note that the "related example" may not be as related as you think. In the sentence

A lo que yo temo es a la maldita casualidad

the object ("casualidad") is not a person, and even so it carries an a. What happens here is that the verb temer, with the sense of "to fear something" can be used in Spanish both as a transitive or intransitive verb. In the latter case, you will always need the a preposition. You can tell if the verb is being used as transitive or intransitive by the use of la/le/lo:

Temer(le) a los gatos.

Temer(le) a la oscuridad.

Temer(le) a la casualidad.

But:

Temer a los gatos > Temerlos.

Temer la oscuridad > Temerla.

Temer la casualidad > Temerla.

The Harry Potter example, nonetheless, is different. The verbal locution echar de menos always acts as a transitive verb. So in this case you will need the personal a:

Echo de menos a mi gato.

Echo de menos la oscuridad.

Echo de menos estar con mis amigos.

But then, what happens with the Harry Potter example? Isn't it talking about his friends? Yes, but (as I hinted in a previous example) it happens that when the object is substituted with la or lo, you don't need the preposition any more:

—¿Echas de menos a tus amigos?

—Sí, los echo de menos.

So, as the sentence starts with a lo, the preposition is not needed:

Lo que de verdad echaba de menos era a sus mejores amigos.

Finally, note that if you make sus mejores amigos the subject of the sentence, you don't need the preposition either:

Sus mejores amigos eran lo que de verdad echaba de menos.