a start

sentence = "there is a wild rose"

letters = sentence.gsub(' ','').split(//)

basic

letters_counter = {}

letters.each do |letter|
  letters_counter[letter] ||= 0
  letters_counter[letter] += 1
end

each_with_object

letters_counter = letters.each_with_object({}) do |letter, a|
                    a[letter] ||= 0
                    a[letter] += 1
                  end

hashnew

letters_counter = letters.each_with_object(Hash.new(0)) do |letter, a|
                    a[letter] += 1
                  end

result

require 'yaml'
puts letters_counter.to_yaml

RESULT: 

---
t: 1
h: 1
e: 3
r: 2
i: 2
s: 2
a: 1
w: 1
l: 1
d: 1
o: 1

nice.
variation:

letters_counter = sentence.each_char.inject( Hash.new(0) ) { |h, letter| h[letter] += 1; h }

sentence.downcase.scan(/[a-z]/).each_with_object(Hash.new(0)) {|letter, counts| counts[letter] += 1}

Using downcase.scan(/[a-z]/) avoids other non-letter characters besides space.