fsharp1

gistfile1.fs

let cleanText (lines: string[]) =
    lines
    |> Seq.skipWhile (fun str -> not (bookStarted str))
    |> Seq.takeWhile (fun str -> not (bookFinished str))
    |> String.concat " "
    |> Seq.map (fun c -> 
        if Char.IsLetter c then Char.ToLowerInvariant c
        else ' ')

// ---->        

let cleanText2 : string[] -> char seq = 
    Seq.skipWhile (not << bookStarted)
    >> Seq.takeWhile (not << bookFinished)
    >> String.concat " "
    >> Seq.map (fun c -> 
        if Char.IsLetter c then Char.ToLowerInvariant c
        else ' ')

My choice:

let cleanText lines =
    lines
    |> Seq.skipWhile (not << bookStarted)
    |> Seq.takeWhile (not << bookFinished)
    |> String.concat " "
    |> Seq.map (fun c -> 
        if Char.IsLetter c then Char.ToLowerInvariant c
        else ' ')

I think this is more readable for people, because it shows data flow (what is important) instead of beauty of function composition and explicit type annotation.