There are many good books in mathematics that can be read for leisure purpose. These books have the feature of simply being "easy" to read, in the sense that even without paper or pen, things can be worked out at ease in our head.
However, there are rarely "leisure" books on advanced topics. A book at the level of, say, GTM, is likely to be very brief and concise on straightforward proofs and computations, with many things left to the reader. Although I have said "straightforward", this can actually put a lot of burden on our mind. To just give one example, for the $L^p$ and $H^s$ space, there are different relations between exponents, involving expressions like $\frac{2n}{n-2s}$ or $1/p+1/q,$ and we often need to word out the value of one such exponent from others. In a book, we just omit such computation since it is just high school algebra. However it still require pen and paper to work things out, and sometimes, one lot oneself by doing this simple calculation again and again.
There are of course a lot of reasons why reading actively and filling the gaps in computation and proofs is beneficial. However, it is equally important to take breaks from such intensive way of learning.
What are some "advanced" books that can be read for "leisure" purpose? That is, (when necessary) even "easy" proofs/computations are giving in an explicit way to increase readability.