Libelli is in the genitive of quantity, used after a noun of quantity to specify "of what." In addition to strict quantities like libra ("pound"), Latin uses this genitive with indefinite quantities, often "substantivized adjectives or pronouns in accusative or nominative," e.g. quid, multum, plus, nihil, etc.
A classic example of this construction is: "Quid novi?" = "What's new?" (lit.: "What of new?")
Here is an example from Cicero that uses quidquid:
quicquid malefici, sceleris, caedis erit, proprium id Rosciorum esse debebit. (Cic. Rosc. Am. 42, 122)
Whatever evildoing, crime, or violence there is, the Roscii surely will have a hand in it.
Here's a translation broken up into parts:
Quare, habe tibi - "Therefore, please accept..."
quidquid hoc libelli - "...whatever book this is..."
qualecumque - "...as it is."