Language Books
Devavanipraveskia: An Introduction to the Sanskrit Language.
I have heard that this is an excellent introduction into Sanskrit. I do not know how much linguistic knowledge is assumed.
An Anglo-Saxon Primer, with grammar, notes and glossary.
I read this book on and off in the fall of 2019 and was thoroughly impressed. It uses grammar-heavy pedagogy and is best for people who have experience learning another inflected language, especially Latin or Ancient Greek. The author prefers to refer to old English as Anglo-Saxon, but I do not remember why.
An Old Icelandic Primer, with grammar, notes and glossary.
This book is written by the same scholar who wrote the Anglo-Saxon primer. I have not used it myself yet but trust that it is similar in structure and pedagogy as the Anglo-Saxon primer. The author prefers to refer to Old Norse as Old Icelandic, which are generally interchangable terms in scholarship as is.
The Lingua Latina per se Illustrata Series
The Lingua Latina per se Illustrata (LLPSI) is the pinnacle of not just Latin learning but language learning in general. It employs the method of comprehensible input and output to teach Latin only through Latin. All grammar explanations, vocabulary definitions, and exercises are written in Latin. The only English you will find is on the backcover and the copyrightpage. It is such a finely graduated reader that if you speak an European language, you are able to read the first chapter, and if you have read the first chapter, you can read the second, and so forth. You will go from such basic sentences as "Roma in Italia est" to select versus of Latin poetry in the very first volume. You can read more about the series here
Mathematics Books
Introduction to Mathematical Cryptography
I read through perhaps once ever semester in college and enjoyed it.
Heavenly Mathematics: The Forgotten Art of Spherical Trigonometry
I love books like this, which gives an enjoyable introduction to an interesting area of math. It is thorough enough to leave you able to work out some problems in spherical trignometry, but not so rigorous that it feels like you are reading a textbook.