It is in very good condition, with only a few tears on the cover and spine.
That unfortunately is a little bit a of a contradiction, as far as a valuation of a book is concerned: once there is cover or spine damage, a book is not considered in “very good” condition. A lttle rubbing or wear on a cover is to be expected, and even slightly rounded corners might allow a book to be considered very good - tears however are never good, and damage to the spine is a significant factor, as the condition of the spine can point to how well the inside pages are bearing up.
In the case of a 1959 Unicorn
the fact that the spine is torn will really affect the perceived value of the copy, as the cloth bound spine is a large part of why people want to collect those early editions.
The later cardboard spines also don’t wear well, so the cloth ones might even be thought to be more durable, so a damaged cloth spine suggests a book that has had a hard life.
That’s just a set of guide lines – nothing is ever hard or fast, and it’s best to think of value as relative (as ever, what someone who wants the item is prepared to pay). I’ve said befor that I think issuing hard-and-fast pricing is a speculators charter, so I shy away from anything definitive, but based on what I’ve seen, I’d imagine that £30 to £50 is a fair price for an undamaged copy (remember that although it’s old, it was printed in fairly large quantities, as far as can be told, and isn’t particularly scarce), so it might get the lower estimate if the copy isn’t too badly damaged. You might be lucky, and find someone prepared to pay more.