RAW states that: "A wizard also selects a number of additional 1st-level spells equal to his Intelligence modifier to add to the spell book".
From this sentence alone, it seems clear that
- the INT based spells are 1st level only
- if INT changes, the number changes
But the sentence uses the words "also" and "additional" which refer you back to the previous sentence, which starts with "A wizard begins play with". This language is somewhat unique and in context it could be interpreted to mean "At first level a wizard gets a spell book with a number of 1st-level spells equal to 3 plus his INT modifier at the time he takes his first wizard level."
The question is whether the unique wording suggests something unique in the RAI or it is just leftover language that wasn't cleaned up. The 3.5 language isn't significantly different, and IMO the intent was to represent a pre-adventuring lifetime of study where a wizard with high INT could leverage it to get a slight advantage at first level.
I think the hang up with "all means all" here (I'm not the only one) is that spells in a spell book represent something physical and tangible that a character would otherwise have to pay and spend time to have (and might seem difficult to abstract away if the bonus goes away). But if you get free spells when levels increase then why not also when INT increases?
The "begins play" in the first sentence leads one to believe this is a something unique that you only get at 1st-level. If you interpret that the language simply didn't consider later INT increases, you could argue that "1st level spells" in the second sentence should be "spells of the highest level you can currently cast, or lower".
I guess for simplicity's sake I'll consider the list of spells in the book to be an abstraction which moves up or down with INT and/or level. And to match RAW the extra spells from INT will be 1st-level, even though they weren't much to gain at levels 8-10 when INT modifier increased. That will net the character 3 more spells or the equivalent of 45 gp in borrowed spell books and inks it would otherwise cost for those spells.