I'm trying to identify a science fiction novel I've read in my teens. Unfortunately, I don't remember a lot about it. Here's what I remember:

* It was hard science fiction.
* It featured space travel.
* It had an overt mystery / suspense tone, with an idea of larger / alien force at work.
* It featured a man, who (I believe) went on a space journey, and is coming back to Earth with a horrible realisation of something he has found (which, as far as I remember, isn't spelled out completely).
* I remember a particular scene where the narrator (who is also the man) speaks about his (former) life on Earth, describing his home with ocean view in San Francisco (or otherwise a coastal area of California, but it could have been the east coast too).
* I read it during the late 90s, roughly 1995-2000.
* The cover art mostly had blue colours, with some kind of space vessel (in black or otherwise dark colours) depicted on it.

I'm sorry this sounds all very vague, but I don't remember much further. Does it ring a bell to anyone?

Edit: at the request of some, I'm adding here other random bits for the correctness of which I wouldn't vouch and which could be completely wrong or unrelated to the novel. While I'm *relatively* certain about the above, the following should be taken with a grain of salt:

* I believe that the novel also came out during that time period, as I bought it at a local book store browsing along (then) contemporary science fiction works.
* I believe that the narrator is writing to a female, possiblity his lover, who is expecting his return. I.e. there is some relationship/romanic side, though it is not the major point of the story. Probably just a means to tell the story.
* I (think I) remember vaguely that the discovery the narrator made was related to him being manipulated in some way (genetically or otherwise mentally) by the aforementioned higher force. It is possible that on the ship he's returning on, he's carrying along that force in some way. While there was no explicit 'end of the world' / 'destruction of humanity' theme, it is hinted that the man returning home is not in any way the man who set out on the journey.
* In the scene where the beach house is mentioned, I believe the ocean is compared to the vastness of the universe in quite a dark tone.
* Exploration might have been the reason the man went into space. He flew alone, at speeds faster than light, and the journey was long, lasting a few light years at the least.

Again, I might be confusing all of these with odd bits from other science fiction works. For the record, I'm mostly a fantasy reader (Wheel of Time, The Seventh Gate) and have not read a lot of pure science fiction (bar a few collections of short stories from Nebula Award winners). You could contend I should remember it better, but as noted it's been a while and the strongest lingering memory I have from this story is this ominous feeling of discovering something horrible about the universe.

Edit 2: added a few unconfirmed points resulting from answers to questions posed here. I will continue to update the entry as I add answers and note this at the bottom in case you've already read the original question.