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I think it's fine for beginners to underuse Dr. Marten's book a bit.
Literal
Beginner [topic-は] Dr. Marten [attributive-の] book [object-を] not-use-[too-much-なさすぎ]-[te-even-if-ても] good [quotative-と] think.
A tricky but instructive construction. ~なさすぎる ('to do too little of X') is the under-side of ~すぎる ('to do too much'). It's formed from the negative stem ~な+さ (nominalizing the な-adjective-like negation) + すぎる. So 使わない + さすぎる → 使わなさすぎる 'use too-little-of.' ~てもいい is the 'it's okay to X' pattern. The resulting meaning: 'I think it's fine to not use the book too much.'