If you enjoyed the Hitchhiker's Guide book, you might also enjoy Terry Pratchett's Discworld books, clean British fantasy humor. I think there's a thread in this forum about them, you'll see different reading orders suggested, but they're all good.
I second the recommendation of Three Men in a Boat and the sci-fi "sequel." If you're interested in classics, I've recently been reading Edgar Rice Burroughs' Mars series (A Princess of Mars is the first) and quite enjoying them. The first five can be found at Project Gutenberg (gutenberg.net) for free since they're out of copyright. Edit:
They can also be found in the collections here at mobileread,
https://www.mobileread.com/forums/showthread.php?t=54599 (this is the epub link). Don't know how I missed them the first time I looked.