Disclaimer: this is the review of MEAP version - all chapters are already in place, some appendixes are missing & book clearly needs A LOT of editing. It feels visibly more "rough" & "unbaked" than majority of MEAPs at this stage.
I was never really a big fan of F#. Maybe just because it's one of very few languages I've started to use without reading a decent intro book first - I've just sit down & started scribbling one day: not really my way of doing stuff. Additionally, I'm not an advocate of hybrid OO & FP languages (like Scala or F#) - there's nothing wrong in having freedom of working out your own mixed style, but reality can be really brutal & I've seen what rough pragmatism can do to idealistic beliefs of over-enthusiastic engineers.
Nevertheless ... Let's get back to the book itself. It's good. It's surprisingly good. Why? Because it tries to teach you IDIOMATIC F#. It doesn't try to cover everything, it doesn't care about .NET platform itself, neither various types of products one can make. Nope. Nothing of that kind. What's even more cool, it doesn't try to teach you what's variable or namespace. What it does instead it telling you (by example) "the way of F#" -> the concepts & principles that belong to the foundation of F# language. It worked quite well in my case - I was thinking about the F# code "out of the C# box" - maybe even for the first time.
What did I like most? Not the "usual suspects" (Type Providers, Type Providers, enough of Type Providers! The same for pipelines & REPL), but a decent intro into F# collections, great chapter on modelling relationships (a lot of new stuff for me), really good chapter on Options, short (but good) intro to Paket.
What didn't I like? Capstone chapters failed to catch my interest. Testing chapters felt too brief (especially FsCheck) - but there were some very good remarks, e.g. about TDD or mocking in F# & whether it makes sense or not.
What can I say more? It's a really good book. Most likely you'll do better by grabbing Don Syme's one first (Expert F#), but this one has one big advantage - it's not complete by any means, but it's idiomatic to the bone, which makes it more likely to ignite the love to the F# in the reader. Did it work that way in my case? ... Well, no, it didn't :) I'm still deeply in love with Elixir, but since today I respect F# significantly more :)