I started reading the books because of the “shit film,” which I liked a whole awful lot. I haven’t seen it recently, so I’m not sure if it would hold up for me now. The first movie wasn’t so much an adaptation of the book as it was a Chevy Chase vehicle using a pretty good book as its structure; Chase’s schtick boosts up the more unbelievable or slow parts of the book, while the mystery story keeps it from being another weak SNL-alumnus movie that just jumps from comedy sketch to comedy sketch.
The second movie was deplorable, and IIRC it wasn’t even loosely based on any of the books, just the character.
With that out of the way: I recommend at least one of the Fletch books. When you read them all back-to-back as I did, the central gimmick just starts to seem less clever and more blatant. The central gimmick is (not sure if it’s really a spoiler, and it should be somewhat obvious after seeing the movie, but don’t read if you like to be completely surprised):
Each book starts out with two plot lines that seem completely separate, but manage to converge by the end.
Even after figuring out the gimmick, there was one book where it took me by surprise. (I won’t say which one, so as not to ruin the surprise and because I can’t remember which one.)
I remember Fletch, Too and Confess Fletch being my favorites of the series. Fletch Won wasn’t too bad, but IIRC it was the end of the series and it was already feeling a little tired. I really hated Fletch’s Moxie and Carioca Fletch. I went on to read one or two of the Flynn books, the off-shoot series, but had already lost interest by that point.
Bottom line is that they’re pretty conventional mystery stories, with an engaging gimmick and a pretty interesting central character. The movie was basically true to the spirit of Fletch’s character, but with a lot of Chevy Chase’s goofy screwball comedy grafted onto it. It was “irreverent,” but in the 80’s sense – they play around with the conventions of mystery novels but it’s not as if they were stunning examples of post-modernism. If you’re a fan of mystery series or are on the fence, you’d probably like them. I strongly doubt they’re good enough to convert anyone, though.