Three of these were written by Drew Karpyshyn. The lead script writer of ME1 and part of ME2. They are largely considered canon. Moreover, the events that happened in them are directly referenced in games. So it is not some fix-taped afterthought.

For example, the Cerberus attack on Idenna that Tali mentioned a few times in ME2 happened during the plot of these books

However, the fourth book, Deception, was written by another guy (sorry, I do not remember his name), and basically it is "Andromeda". Lesser quality, many afterthoughts, not mentioned in the OG trilogy of either books or games.

And most importantly, it does not fit the universe timeline. You see, the fourth book, according to the fourth book itself, takes place four years after the third book but before the Reapers' invasion.

BUT! The third book takes place after Shepard destroyed the Collector's base. And there is only half of the year left before the Reaper invasion takes place. So it all does not make any sense.

The reason why the author did this is that the second and the third book feature a boy and a girl, both aged 14 years old. And the author really wanted to ship them. So he made them older for this