Hilbert and Courant is BY FAR the best mathematical physics book in existence. No contest. Boas and all the others are good, even very good. H & C beats them by a kiloParsec.
Hildebrand "Methods of Applied Mathematics" is the most approachable math text I've read. After hearing me express confusion about various topics that my dad thought were straightforward, he gave me his book from when he was in college, and yes, things were suddenly straightforward.