Neville Coleman's "Nudibranchs Encyclopedia", while a bit old, is an excellent book, well worth having together with Gosliner's books.
Thosee by Helmut Debelius are also well worth having.
I found that the more books I have on sea life, the more I enjoy them, they are an excellent investment. Their authors have done us all lovers of sea life a huge favor by gathering and publishing their knowledge, we owe them a huge debt of gratitude.
Through my late dive buddy Capt Jim Black, an expert nudibranch aficionado, I had the opportunity to meet several of these authors, and they were a pleasure to be with. Their books are on my desktop as I write this.
Amazon offers an excellent variety of books on reef life:
https://www.google.com/search?q=amazon.com+nudibranch+reef+life+books&sca_esv=