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That's simply to make the sentence more polite. The -たい-form of a verb acts like an い-adjective, which also explains its negative (早い - 早くない / 読みたい - 読みたくない). With the -たい form you may use が instead of を which should make things more recognizable:
この本が読みたいです (This book is want-to-readable; similar mechanism to 好き "likeable")
You can also flip the order: 読みたい本 "the book that I want to read".
More in general, you may see verbs in the short form followed by です which again simply makes the sentence more polite. For example, while the polite form of 早かった would usually be 早いでした, 早かったです is also valid.
Looking up 早いでした on Google does turn up quite a lot of results - on websites that are clearly not created by foreigners - but now that I look more closely, they are mostly blog posts and comments where loose grammar can be expected.