Where the Words Break
or Why Translators Like Kanji
"I want to buy a manga to help me learn my kana. What do you suggest?"
My very first attempt at translating a manga was before I had taken any courses or even picked up any of those Learn Japanese in 10 Minutes a Day books. I had a friend's first-year Japanese textbook, another friend's ancient version of the Green Goddess Dictionary, a photocopied kana chart and a copy of the Japanese cine-manga for Orguss. In the first thirty pages of text, I managed to translate one word balloon! It was useful in that it told me just how much there was to learn before I could actually translate Japanese at any level.
And it taught me my first lesson in the difficulties of Japanese: We don't know where one word ends and the next one begins.
Aside from some early-reader books (for children from about 4 to 7 or 8 years old), all of Japanese runs continuously with one word bumped up to the next. There are no breaks between words such as there are in English and other Western languages.
So if, at an early stage, you start to try to translate Japanese, you start with the first syllable and look it up in the dictionary. If nothing seems to make sense with respect to the pictures, you add on the second syllable and look up the two syllable combination. Then you add on the third syllable, and so on. But you're a novice, so you may find two or three words that actually do make sense, and you don't have enough experience with Japanese to judge yet which one is right.
Yes, it is an awful thing to be an early novice and want to translate Japanese.
Sure every 1st year Japanese student has pondered that it would be so nice if they didn't have to learn all of those kanji -- or even better, if Japan could adopt our alphabet! Wouldn't that be great?
No.
There are so many homophones in Japanese that putting a word like "kankou" (my Green Goddess lists 16 different meanings) in romaji means that we translators would have a very hard time figuring out which meaning it is! But all of those 16 meanings have different kanji when in modern-day written Japanese, so the kanji lead you directly to the correct meaning. Also kanji break up the words in a sentence. Whenever you see a kanji, it signals the start of a new word. (There are exceptions such as the honorific "o" that comes at the start of some words can be in kana with kanji following it, but generally the kanji will start a new word.)
So kanji both work to break up a sentence and pin down the meaning. By your second or third year of Japanese, they'll be a godsend to your understanding of the language. Oh, you'll still hate them -- you have to learn nearly 2000 of them after all -- but you won't wish they didn't exist anymore.
In answer to the top question, you might as well get yourself a children's book where they actually put spaces between the words. It won't help you much since you'll still have a very hard time figuring out what's being said, but it'll help more than a manga will.