Siempre miro el escaparate de la librería antes de entrar, por si veo algún libro interesante.

Spanish often leaves out subject pronouns because the verb ending already shows who is doing the action.

• miro = I look
• veo = I see

So yo is not necessary here. You could say Yo siempre miro..., but that would usually add emphasis, contrast, or a more personal tone.

Putting siempre first emphasizes the idea of habit: I always...

Spanish adverbs like siempre are fairly flexible in position. For example, these are both possible:

• Siempre miro el escaparate...
• Miro siempre el escaparate...

The version with siempre at the beginning sounds very natural and slightly highlights the routine.

Because they do not mean exactly the same thing.

• mirar = to look at, deliberately direct your eyes toward something
• ver = to see, notice or perceive something

So:

• Siempre miro el escaparate... = I deliberately look at the window display
• ...por si veo algún libro interesante = in case I happen to see an interesting book

This is a very natural contrast in Spanish.

In Spain, escaparate usually means the shop window or display window at the front of a store, where items are arranged for people outside to see.

So el escaparate de la librería is the bookstore’s window display.

No. This is a very common false friend.

• librería = bookstore / bookshop
• biblioteca = library

So in this sentence, the person is looking at the window of a bookstore, not a library.

Here de links the two nouns and shows association or possession:

• el escaparate de la librería = the bookstore’s display window

If you used en la librería, that would mean in the bookstore, which expresses location instead of ownership/association.

After antes de, Spanish normally uses the infinitive when the subject is the same as in the main verb.

So:

• Siempre miro... antes de entrar = I always look... before entering / before I go in

This is equivalent to saying before I go in in English.

Yes, but it changes the structure.

• antes de entrar is used when the subject is the same person as the main verb.
• antes de que entre is used when you introduce a finite clause, often with a potentially different subject, and it requires the subjunctive.

Examples:

• Miro el escaparate antes de entrar.
  Same subject: I look before entering.

• Miro el escaparate antes de que entre mi amigo.
  Different subject: I look before my friend goes in.

In your sentence, antes de entrar is the simplest and most natural choice.

Por si means in case.

It introduces a possible situation that motivates the action in the main clause.

So:

• Siempre miro el escaparate... por si veo algún libro interesante
  = I always look at the window display... in case I see an interesting book

The idea is not just if, but I do this as a precaution / because it might happen.

Because por si is normally followed by the indicative, not the subjunctive.

Examples:

• Lleva un paraguas por si llueve.
• Te llamo por si necesitas ayuda.
• Miro el escaparate por si veo algo interesante.

Even though English often uses forms like in case I see, Spanish still uses the indicative here because it presents the situation as a real possibility.

Algún here means something like some or any.

• un libro interesante = an interesting book
• algún libro interesante = some interesting book / any interesting book

In this sentence, algún sounds more natural because the speaker is not thinking of one specific book, just the possibility of spotting one.

Because alguno changes form before a masculine singular noun.

• alguno → algún before a masculine singular noun
• algún libro
• algún problema

This shortening is called apocopation.

Compare:

• algún libro
• alguna revista
• alguno es interesante
  Here it stays alguno because it is not directly before the noun.

In Spanish, descriptive adjectives usually come after the noun.

So:

• libro interesante = interesting book

That is the normal order. Putting the adjective before the noun is sometimes possible, but it often sounds more literary, emphatic, or changes the nuance. In everyday Spanish, algún libro interesante is the natural phrasing.

It is very natural here because por si veo algún libro interesante is an added clause explaining the reason or motivation for the previous action.

So the comma helps separate:

• main idea: Siempre miro el escaparate de la librería antes de entrar
• added explanation: por si veo algún libro interesante

In informal writing, some people might omit it, but with the comma the sentence is very clear and well punctuated.